Display device and electronic device

ABSTRACT

It is an object of the present invention to provide a display device in which images can be seen under a dark place to intense external light. In the display device, display is performed by changing the gray scale number depending on external light intensity, and display modes can be switched depending on contents displayed on the screen. An analog mode and a digital mode are switched depending on external light intensity. In an analog digital switching circuit, when a video signal is an analog value, a signal is outputted to a pixel array without any change and, when the video signal is a digital value, the signal is outputted to a circuit that performs a digital operation such as a latch circuit. Consequently, display gray scales of a pixel are changed appropriately. Accordingly, a clear image can be displayed. For example, it is possible to ensure visibility in a wide range of a dark place or under indoor florescent light to outdoor sun light.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device equipped with a screenthat displays a character, a still image, a moving image, or the likeand relates to a technique to improve visibility of a display screen invarious environments where the display device is used.

2. Description of the Related Art

In recent years, a so-called self-luminous type display device having apixel that is formed of a light-emitting element such as alight-emitting diode (LED) has been attracting attention. As alight-emitting element used for such a self-luminous type displaydevice, an organic light-emitting diode (OLED) (also called an organicEL element, an electro luminescence: EL element, and the like) has beendrawing attention and used for an EL display (for example, an organic ELdisplay or the like). Since a light-emitting element such as an OLED isa self-luminous type, it has advantages such as higher visibility ofpixels than that of a liquid crystal display, and fast response withoutrequiring a backlight. The luminance of a light-emitting element iscontrolled by a current value flowing through it.

As a driving method for controlling a light-emitting gray scale of sucha display device, there are a digital gray scale method and an analoggray scale method. According to the digital gray scale method, alight-emitting element is turned on/off in a digital manner to express agray scale. Meanwhile, the analog gray scale method includes a methodfor controlling the light-emitting intensity of a light-emitting elementin an analog manner and a method for controlling the light-emitting timeof a light-emitting element in an analog manner.

In the case of the digital gray scale method, there are only two states:a light-emitting state and a non-light-emitting state. Therefore, onlytwo gray scales can be expressed if nothing is done. Accordingly,another method is used in combination to achieve multiple gray scales. Atime gray scale method is often used as a method for multiple grayscales (see Reference 1: Japanese Patent Application Laid-Open No.2001-324958 and Reference 2: Japanese Patent Application Laid-Open No.2001-343933).

As a display for controlling a display state of a pixel in a digitalmanner and expressing a gray scale in combination with a time grayscale, there are some displays as well as an organic EL display usingthe digital gray scale method. As an example, there is a plasma displayor the like.

The time gray scale method is a method for expressing a gray scale bycontrolling the length of a light-emitting period or the frequency oflight emission. In other words, one frame period is divided into aplurality of sub-frame periods, each of which is weighted with respectto the frequency of light emission and a light-emitting period, and thenthe total weight (the sum of the frequency of light emission and the sumof the light-emitting period) is differentiated for each gray scale,thereby expressing a gray scale.

In the meantime, an apparent image quality is emphasized even in such adisplay panel and many display panels provided with a function to adjustbrightness or contrast automatically or manually prevails widely. Forexample, a liquid crystal display device provided with adjustability toimprove visibility of a gray scale by changing transmittance of a liquidcrystal without increasing luminance of a backlight of a liquid crystalpanel is known (see Reference 3: Japanese Patent Application Laid-OpenNo. 2003-186455).

However, although a liquid crystal display panel has preferablevisibility in an indoor environment of 300 to 700 luxes, there is aproblem that the visibility is deteriorated significantly in an outdoorenvironment of 1,000 luxes or more. There is a liquid crystal panelhaving a structure where external light is reflected by a pixelelectrode, which is referred to as a reflective liquid crystal panel;however, the image quality is rather deteriorated under indoorflorescent light; thus, the problem is not solved fundamentally. Inother words, it is not solved to ensure visibility in wide range ofunder dark or indoor florescent light to outdoor sun light.

SUMMARY OF THE INVENTION

Thus, it is an object of the present invention to provide a displaydevice in which images can be seen under a dark place to intenseexternal light.

According to one feature of a display device in the present invention,where a plurality of pixels is arranged in matrix, the display devicehas a source driver and a gate driver, and the source driver has acircuit that supplies any one signal of a digital value and an analogvalue to the pixel depending on external light intensity.

According to another feature of a display device in the presentinvention, where a plurality of pixels is arranged in matrix, thedisplay device has at least two display modes, an analog signal issupplied to the pixel in the first display mode, a digital signal issupplied to the pixel in the second display mode, and the display modesare switched depending on external light intensity.

According to another feature of a display device in the presentinvention, where a plurality of pixels is arranged in matrix, thedisplay device has at least first and second display modes, the pixelhas a light-emitting element, an analog signal is supplied to the pixelin the first display mode, a digital signal is supplied to the pixel inthe second display mode, the display modes are switched depending onexternal light intensity, and voltage supplied to the light-emittingelement is different in the first and second display modes.

According to another feature of a display device in the presentinvention, where a plurality of pixels is arranged in matrix, thedisplay device has at least first and second display modes, the pixelhas a light-emitting element and a transistor, a first electrode of thelight-emitting element and one of source and drain electrodes of thetransistor are connected, an analog signal is supplied to the pixel inthe first display mode, a digital signal is supplied to the pixel in thesecond display mode, the display modes are switched depending onexternal light intensity, and voltage between a second electrode of thelight-emitting element and the other of the source and drain electrodesof the transistor is different in the first and second display modes.

Note that, according to the present invention, a transistor of variousmodes can be applied. Thus, a kind of transistor that can be applied isnot limited. Therefore, a thin film transistor (TFT) using a non-singlecrystalline semiconductor film typified by amorphous silicon orpolycrystalline silicon, a MOS transistor which is formed using asemiconductor substrate or an SOI substrate, a junction transistor, abipolar transistor, a transistor using a compound semiconductor such asZnO or a-InGaZnO, a transistor using an organic semiconductor or acarbon nanotube, or other transistors can be applied. Note that hydrogenor halogen may also be contained in the non-single crystallinesemiconductor film. In addition, various substrates can be used as akind of substrate where a transistor is disposed and the substrate isnot limited to a specific one. Thus, a transistor can be disposed over asingle-crystalline substrate, an SOI substrate, a glass substrate, aquartz substrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone material substrate, or the like, for example.Moreover, a transistor may also be formed over one substrate andthereafter the transistor is moved to another substrate to be disposedover another substrate.

Note that various modes can be applied as a structure of a transistor,which is not limited to a specific structure. For example, a multi-gatestructure where the number of gates is two or more may also be employed.By employing the multi-gate structure, off current can be reduced,withstand pressure of a transistor can be improved to have preferablereliability, and a flat characteristic can be obtained with few currentbetween a drain and a source changed even when voltage between the drainand source is changed when a transistor is operated in a saturationregion. In addition, a structure where a gate electrode is disposedbetween upper and lower channels may also be employed. By employing thestructure where a gate electrode is disposed between upper and lowerchannels, a channel region increases; therefore, a current value can beincreased and a preferable S value can be obtained because a depletionlayer is formed easily. Moreover, any one of a structure where a gateelectrode is disposed over a channel, a structure where a gate electrodedisposed below a channel, a forward stagger structure, and a reversestagger structure may be employed. Alternatively, a channel region maybe divided in a plurality of regions, or a channel region may beconnected in parallel or in series. Further, a source or drain electrodemay be overlapped with a channel (or part thereof). By employing astructure where a source or drain electrode may be overlapped with achannel (or part thereof), an electric charge is stored in part of achannel, which can prevent the operation from being unstable.Furthermore, there may also be an LDD region. By providing the LDDregion, off current can be reduced, withstand pressure of a transistorcan be improved to have preferable reliability, and a flatcharacteristic can be obtained without current between a drain and asource changed so much even when voltage between the drain and thesource is changed when a transistor is operated in a saturation region.

Note that, according to the present invention, “being connected”includes a case electrically connected and a case directly connected.Therefore, in a structure disclosed by the present invention, inaddition to a predetermined connection relation, other elements thatenables electrical connection therebetween (for example, a switch, atransistor, a capacitor element, an inductor, a resistive element, adiode, or the like) may also be disposed. Alternatively, there may bedirect connection and disposition without interposing other elementtherebetween. Note that a case only including a case where there isconnection, without interposing other element that enables electricalconnection therebetween, and direct connection does not include a casewhere there is electrical connection. Such a case is to be described as“being directly connected.” Note that a case described as “beingelectrically connected” includes a case of being electrically connectedand a case of being directly connected.

Note that, according to the present invention, one pixel shows oneelement that can control brightness. Thus, as one example, one pixelrefers to one color element, and brightness is expressed by only onecolor element. Therefore, in a case of a color display device includingcolor elements of R (red), G (green), and B (blue) at that time, aminimum unit of an image includes three pixels of R, G, and B. Note thatthe color element is not limited to three colors and three or morecolors may be used. For example, there are R, G, B, and W (W is white),R, G, and B added with yellow, cyan, and magenta, and the like. Inaddition, as another example, in a case where brightness of one colorelement is controlled by using a plurality of regions, one of theregions is considered as one pixel. Thus, as one example, in a case ofperforming an area gray scale, there is a plurality of regions wherebrightness is controlled per one color element and a gray scale isexpressed in the whole region, and one region where brightness iscontrolled is considered as one pixel. Thus, in this case, one colorelement includes a plurality of pixels. In addition, in this case, thesize of a region that contributes to display may be different dependingon a pixel. Moreover, in a region where a plurality of brightness iscontrolled per one color element, that is, a plurality of pixels thatconstitutes one color element, a viewing angle may be expanded so that asignal supplied to each is slightly made different.

Note that, according to the present invention, a pixel includes a casewhere pixels are arranged in matrix. Herein, “pixels are arranged inmatrix” includes a case of a so-called lattice arrangement in which aperpendicular stripe and a horizontal stripe are combined with eachother, a case where dots of three color elements have a so-called deltaarrangement when full color display is performed using three colorelements (for example, R, Q and B), and further a case of Bayerarrangement. In addition, the size of a light-emitting region thereofmay be differed in each dot of a color element.

Note that a transistor refers to an element having three terminals eachincluding at least a gate, a drain, and a source. The gate refers to thewhole of a gate electrode and a gate wiring (also referred to as a gateline, a gate signal line, or the like), or part thereof. The gateelectrode refers to a semiconductor to form a channel region, an LDD(Lightly Doped Drain) region, or the like, and a portion of a conductivefilm which is overlapped through a gate insulating film. The gate wiringrefers to a wiring to connect between gate electrodes of each pixel orto connect to a wiring different from the gate electrode.

However, there is a portion that serves as a gate electrode and a gatewiring. Such a region may be referred to as a gate electrode or a gatewiring. In other words, there is a region where a gate electrode and agate wiring cannot be distinguished apparently. For example, when thereis a channel region to overlap with a gate wiring that is arranged bybeing extended, the region serves as a gate wiring as well as a gateelectrode. Thus, such a region may be referred to as a gate electrode ora gate wiring.

In addition, a region formed from the same material as the gateelectrode and connected to the gate electrode may also be referred to asa gate electrode. In the same manner, a region formed from the samematerial as the gate wiring and connected to the gate wiring may also bereferred to as a gate wiring. In such a region, in a strict sense, thereis a case where the region is not overlapped with a channel region or afunction to connect to another gate electrode is lacked. However, withrelation to a manufacturing margin or the like, there is a region formedfrom the same material as the gate electrode or the gate wiring andconnected to the gate electrode or the gate wiring. Thus, such a regionmay also be referred to as a gate electrode or a gate wiring.

Moreover, for example, a gate electrode of one transistor and a gateelectrode of another transistor in a multi-gate transistor are connectedto a conductive film formed from the same material as the gate electrodein many cases. Since such a region is a region to connect the gateelectrode and the gate electrode to each other, the region may also bereferred to as a gate wiring; however, since the multi-gate transistorcan be regarded as one transistor, the multi-gate transistor may also bereferred to as a gate electrode. In other words, those formed from thesame material as the gate electrode or the gate wiring and arranged bybeing connected thereto may also be referred to as a gate electrode or agate wiring. Further, for example, a portion of a conductive film whichis connected to the gate electrode or the gate wiring may also bereferred to as a gate electrode or a gate wiring.

Note that a gate terminal refers to part of a region of a gate electrodeor a region electrically connected to the gate electrode.

Note that a source refers to the whole of a source region, a sourceelectrode, and a source wiring (also referred to as a source line, asource signal line, or the like), or part thereof. The source regionrefers to a semiconductor region where a P-type impurity (boron orgallium) or an N-type impurity (phosphorus or arsenic) is containedmuch. Therefore, the source region does not include a region where aP-type impurity or an N-type impurity is slightly contained, that is, aso-called an LDD (Lightly Doped Drain) region. The source electrode isformed from a material different from the source region, which refers toa portion of a conductive layer which is arranged by being electricallyconnected to the source region. However, the source electrode mayinclude the source region to be referred to as a source electrode. Thesource wiring refers to a wiring to connect between source electrodes ofeach pixel or to connect a wiring different from the source electrode.

However, there is a portion that serves as a source electrode and asource wiring. Such a region may be referred to as a source electrode ora source wiring. In other words, there is a region where a sourceelectrode and a source wiring cannot be distinguished apparently. Forexample, when there is a source region to overlap with a source wiringthat is arranged by being extended, the region serves as a source wiringas well as a source electrode. Thus, such a region may be referred to asa source electrode or a source wiring.

In addition, a region formed from the same material as the sourceelectrode and connected to the source electrode or a portion to connecta source electrode and a source electrode to each other may also bereferred to as a source electrode. In addition, a portion overlappedwith a source region may also be referred to as a source electrode. Inthe same manner, a region formed from the same material as the sourcewiring and connected to the source wiring may also be referred to as asource wiring. In such a region, in a strict sense, there is a casewhere a function to connect to another gate electrode is lacked.However, with relation to a manufacturing margin or the like, there is aregion formed from the same material as the source electrode or thesource wiring and connected to the source electrode or a source wiring.Thus, such a region may also be referred to as a source electrode or asource wiring.

Moreover, for example, a portion of a conductive film where the sourceelectrode and the source wiring are connected may also referred to as asource wiring.

Note that a source terminal refers to part of a source region, a sourceelectrode, or a region electrically connected to the source electrode.

Note that a drain is the same as the source.

Note that, according to the present invention, the description of “beingformed over a certain object” does not necessarily refer to “being indirect contact with the certain object.” This includes a case wherethere is no direct contact, that is, a case where another object issandwiched therebetween. Therefore, for example, a case where a layer Bis formed over a layer A includes a case where the layer B is formed onthe layer A to be in direct contact therewith and a case where anotherlayer (for example, a layer C, a layer D, or the like) is formed on thelayer A to be in direct contact therewith and the layer B is formedthereon to be in direct contact therewith. In addition, the same can besaid for the description of “above a certain object,” which does notnecessarily refer to “being in direct contact with the certain object,”and a case where another object is sandwiched therebetween is included.Therefore, for example, a case where a layer B is formed above a layer Aincludes a case where the layer B is formed on the layer A to be indirect contact therewith and a case where another layer (for example, alayer C, a layer D, or the like) is formed on the layer A to be indirect contact therewith and the layer B is formed thereon to be indirect contact therewith. Note that the same can be said for thedescription of “under a certain object” or “below a certain object,”which includes a case where there is direct contact and there is nodirect contact.

According to the present invention, it is possible to provide a displaydevice superior in visibility by controlling the gray scale number of adisplay image depending on external light intensity. In other words, itis possible to obtain a display device in which visibility is ensured ina wide range of a dark place or under indoor florescent light to outdoorsun light.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading of the followingdetailed description along with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 2 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 3 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 4 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIGS. 5A to 5D are diagrams each explaining a configuration of a displaydevice according to the present invention;

FIGS. 6A and 6B are diagrams each explaining a configuration of adisplay device according to the present invention;

FIG. 7 is a diagram explaining a driving method of a display deviceaccording to the present invention;

FIG. 8 is a diagram explaining a driving method of a display deviceaccording to the present invention;

FIG. 9 is a diagram explaining a driving method of a display deviceaccording to the present invention;

FIG. 10 is a diagram explaining a driving method of a display deviceaccording to the present invention;

FIG. 11 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 12 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 13 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIGS. 14A and 14B are diagrams each explaining a driving method of adisplay device according to the present invention;

FIGS. 15A and 15B are diagrams each explaining a driving method of adisplay device according to the present invention;

FIGS. 16A and 16B are diagrams each explaining a driving method of adisplay device according to the present invention;

FIG. 17 is a diagram explaining a driving method of a display deviceaccording to the present invention;

FIG. 18 is a diagram explaining a driving method of a display deviceaccording to the present invention;

FIG. 19 is a diagram explaining a driving method of a display deviceaccording to the present invention;

FIG. 20 is a diagram explaining a driving method of a display deviceaccording to the present invention;

FIG. 21 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 22 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 23 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 24 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 25 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 26 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 27 is a view explaining an electronic device to which the presentinvention is applied;

FIGS. 28A and 28B are diagrams explaining a configuration of a displaydevice according to the present invention;

FIG. 29 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 30 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIGS. 31A to 31H are views each explaining an electronic device to whichthe present invention is applied;

FIG. 32 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 33 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 34 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 35 is a diagram explaining a configuration of a display deviceaccording to the present invention;

FIG. 36 is a view explaining a structure of a display device accordingto the present invention;

FIGS. 37A and 37B are views each explaining a structure of a displaydevice according to the present invention;

FIGS. 38A and 38B are views each explaining a structure of a displaydevice according to the present invention;

FIGS. 39A and 39B are views each explaining a structure of a displaydevice according to the present invention;

FIG. 40 is a view explaining a structure of a display device accordingto the present invention;

FIG. 41 is a diagram showing an example of a photo-sensor or anamplifier;

FIG. 42 is a diagram showing an example of a photo-sensor or anamplifier;

FIG. 43 is a diagram showing an example of a photo-sensor or anamplifier; and

FIG. 44 is a diagram showing an example of a photo-sensor or anamplifier.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment Modes of the present invention will be explained below withreference to the accompanying drawings. However, it is to be easilyunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless such changes andmodifications depart from the scope of the present invention, theyshould be construed as being included therein.

Embodiment Mode 1

FIG. 1 shows an overall configuration diagram. A source driver 102 and agate driver 110 are disposed to drive a pixel array 101. Note that aplurality of the source drivers 102 and gate drivers 110 may each bedisposed.

A photo-sensor 113 detects external light (external light that a displaydevice receives). The output is supplied to an amplifier 114. Theamplifier 114 amplifies an electrical signal outputted by thephoto-sensor 113, and the amplified electrical signal is supplied to acontroller 117. Note that the device can be constituted without theamplifier 114 when the electrical signal outputted by the photo-sensor113 is sufficiently large.

The controller 117 controls a display mode switching control circuit2101. A display mode, the gray scale number, or the like is determinedin the display mode switching control circuit 2101. Then, a display modecontrol signal 107 is controlled by being outputted to the source driver102.

The controller 117 controls the display mode switching control circuit2101 based on the signal from the photo-sensor 113. Then, the gray scalenumber of a video signal or the display mode control signal 107 that issupplied to the source driver 102 is controlled by the signal from thephoto-sensor 113, that is, depending on surrounding luminance. Incontrolling the gray scale number, the number may be changed graduallydepending on the surrounding luminance or which display mode a displayis performed in is may be switched by having some display modes.

Accordingly, a display mode, that is, the gray scale number at the timeof displaying is changed, based on the output of the photo-sensor 113.Specifically, when a display device receives strong external light andthe output of the photo-sensor 113 becomes over a certain value, thetotal gray scale number of an image displayed on a display screen isreduced. When the display device receives strong external light, adistinction between a gray scale and another gray scale becomes unclear,and an image displayed on the display screen is blurred. However, byreducing the total gray scale number according to external light whichthe display device receives, as described above, a distinction between agray scale and another gray scale becomes clear and visibility of thedisplay screen can be improved.

In addition, in a case where the total gray scale of an image displayedon the display screen is set to be 2 gray scales by the output of thephoto-sensor 113, although a black display image is usually displayed ona white background image, it may be inverted so that a white displayimage is displayed on a black background image. In such a way,visibility of the display screen can be further improved. Moreover, byincreasing luminance of the white display image, visibility of thedisplay screen can be further improved. The combination of a backgroundimage and a display image is not limited to the white display on theblack background, and arbitrary color combinations can be used as longas the combination can make a contrast (LD ratio is clear) easily.

The output of the photo-sensor 113 is sent to the controller 117 via theamplifier 114. The controller 117 detects whether the output of thephoto-sensor 113 is over a certain value or not. When the output of thephoto-sensor 113 does not reach the certain value, the total gray scalenumber of a video signal outputted to a display panel is not changed. Onthe other hand, when the output of the photo-sensor 113 is the certainvalue or more, the total gray scale number of a video signal outputtedto the display panel is corrected to be smaller.

As shown in Table 1, indoor or outdoor brightness varies according tothe lighting condition, the climate condition such as weather, and time.For example, the illuminance in a room with lighting is approximately800 to 1,000 lux, the illuminance under a cloudy sky of daytime isapproximately 32,000 lux, and the illuminance under a clear sky ofdaytime reaches 100,000 lux.

[Table 1]

A result of comparison among visibilities of a display panel usingelectroluminescence (EL panel), a transmissive liquid crystal panel(transmissive LCD panel), a semi-transmissive liquid crystal panel(semi-transmissive LCD panel), and a reflective liquid crystal panel(reflective LCD) under conditions with such various brightness is shownin Table 2.

[Table 2]

As a result, in an environment with brightness of up to approximately1,500 lux (mainly indoor, such as a hall with lighting), preferablevisibilities are obtained from the EL panel and various liquid crystalpanels except the reflective liquid crystal panel, with any displaypattern (a natural image, text (characters and symbols) and the like).On the other hand, in an environment of 10,000 lux (cloudy daytime), inthe case of the EL panel and the transmissive liquid crystal panel,visibility of part where the contrast is low, such as a half-tone part,tends to be significantly decreased when a natural image is displayed.However, even in this case, visibility of the EL panel is preferable tothat of the transmissive liquid crystal panel. In addition, as for theEL panel, the visibility recovers when the gray scale number isdecreased (from 2 to 8 gray scales), and a visibility practically havingno problem is obtained especially for text display. On the other hand,as for the semi-transmissive liquid crystal panel, even though thecontrast is slightly low in environments of indoor to outdoor overall,preferable visibility is obtained in an environment of 10,000 lux. Thereflective liquid crystal panel is superior in power consumption;however, the visibility tends to decrease in an environment withrelatively low illuminance such as indoor. Since the backlight consumespower, the power consumption of the transmissive liquid crystal panel ishigher than that of the reflective liquid crystal panel. On the otherhand, in the case of the EL panel, the power consumption is lowered in adisplay mode with the reduced gray scale number.

As is apparent from Table 2, by using an EL panel and having a displaymode the gray scale number of which is adjusted according to theexternal light strength, it is possible to provide a display device thevisibility of which is ensured in environments of indoor to outdoor andthe power consumption is lowered.

For example, as for the display device shown in FIG. 1, in a case whereit is detected by the output of the photo-sensor 113 that the displaydevice is receiving external light of from 10 to 100 lux, the total grayscale number is from 64 to 1024 gray scales and not changed. Inaddition, in a case where it is detected by the output of thephoto-sensor 113 that the display device is receiving external light offrom 100 to 1,000 lux, the total gray scale number is corrected to befrom 16 to 64 gray scales by reducing the total gray scale number.Moreover, in a case where it is detected by the output of thephoto-sensor 113 that the display device is receiving external light offrom 1,000 to 10,000 lux, the total gray scale number is corrected to befrom 4 to 16 gray scales by reducing the total gray scale number.Further, in a case where it is detected by the output of thephoto-sensor 113 that the display device is receiving external light offrom 10,000 to 100,000 lux, the total gray scale number is corrected tobe from 2 to 4 gray scales by reducing the total gray scale number.

Note that a selection switch with which a user selects the display modemay be provided for the display device so that the above mode isselected by the user operating the selection switch. In addition, evenin a case where the display mode is selected by the selection switch,the gray scale of the selected display mode may be increased ordecreased automatically depending on the signal of the photo-sensor 113(the external light strength).

Note that the signal line driver circuit or a portion thereof may beconstituted using, for example, an external IC chip in some casesinstead of being provided over the same substrate as the pixel array101.

Note that the amplifier 114 or the photo-sensor 113 may be provided overthe same substrate as the pixel array 101. In this case, the amplifier114 or the photo-sensor 113 may be formed over the same substrate as thepixel array 101. Alternatively, the amplifier 114 or the photo-sensor113 may be disposed over the same substrate as the pixel array 101 byusing COG (Chip On Glass), a bump, or the like.

Note that, as described above, a transistor according to the presentinvention may be any type of transistor, and formed over any substrate.Therefore, the circuits shown in FIG. 1 may all be formed over a glasssubstrate, a plastic substrate, a single crystalline substrate, an SOIsubstrate, or any substrate. Alternatively, part of the circuits in FIG.1 or the like may be formed over one substrate, and the other part ofthe circuits in FIG. 1 or the like may be formed over another substrate.In other words, the whole circuits in FIG. 1 are not necessarily formedover the same substrate. For example, in FIG. 1 or the like, the pixelarray 101 and the gate driver 110 may be formed over a glass substrateusing TFTs, and the source driver 102 (or part thereof) may be formedover a single crystalline substrate, and then an IC chip thereof may beconnected by COG (Chip On Glass) to be provided over a glass substrate.Alternatively, the IC chip may be connected to the glass substrate byTAB (Tape Auto Bonding) or using a printed wiring board.

In the same manner, a photo-sensor according to the present inventionmay be any type of photo-sensor, and formed over any substrate. As anexample of the photo-sensor, a PIN diode, a PN diode, a Schottky diode,and the like can be given. In addition, the photo-sensor may be made ofany material. The photo-sensor may also be formed of amorphous siliconor polysilicon, a single crystal, SOI, or the like. When thephoto-sensor is formed of amorphous silicon or polysilicon, thephoto-sensor can be formed over the same substrate as the pixel arraysimultaneously in the same processing step; thus, the cost can bereduced.

Therefore, the photo-sensor or the amplifier may all be formed over aglass substrate, a plastic substrate, a single crystalline substrate, anSOI substrate, or any substrate. Alternatively, part of the photo-sensoror the amplifier may be formed over one substrate, and the other part ofthe photo-sensor or the amplifier may be formed over another substrate.In other words, the whole photo-sensor or amplifier is not necessarilyformed over the same substrate. For example, in FIG. 1 or the like, thephoto-sensor 113, the pixel array 101, and the gate driver 110 may beformed over a glass substrate using TFTs, and the source driver 102 (orpart thereof) may be formed over a single crystalline substrate, andthen an IC chip thereof may be connected by COG (Chip On Glass) to beprovided over a glass substrate. Alternatively, the IC chip may beconnected to the glass substrate by TAB (Tape Auto Bonding) or using aprinted wiring board.

The source driver 102 can be roughly divided into three portions.

First, a shift register 103 is a circuit that outputs a sequentiallyselecting signal (a so-called sampling pulse). Therefore, the circuit isnot limited to the shift register as long as a circuit that performs asimilar function is used. For example, a decoder circuit may also beused.

A sampling pulse outputted from the shift register is inputted into ananalog digital switching circuit 104. A video signal 106 is inputtedinto the analog digital switching circuit 104 through a video signalline 108. In addition, there is a function, using a display mode controlsignal 107, to control whether the inputted video signal 106 is inputtedinto a digital data processing circuit 105 or inputted into the pixelarray. The display mode control signal 107 is inputted into the analogdigital switching circuit 104 through a display mode control signal line109. For example, when the display mode control signal 107 is an Hsignal or a level of significance, the video signal 106 is inputted intothe pixel array 101. On the other hand, when the display mode controlsignal 107 is an L signal or a level of non-significance, the videosignal 106 is inputted into the digital data processing circuit 105.

The digital data processing circuit 105 has the same function as anormal digital driver. In other words, there is a function to hold thevideo signal 106 and to output the held video signal 106 to the pixelarray.

Therefore, it is possible to switch between a case of inputting thevideo signal 106 into the pixel array 101 without any change and a caseof inputting the video signal 106 into the pixel array 101 after beingtemporarily held by switching the display mode control signal 107.

Thus, it is possible to make the video signal 106 an analog value whenthe video signal 106 can be inputted into the pixel array 101 withoutany change. In this case, it is possible to control a pixel in an analogmanner. Therefore, an analog gray scale method can be used.

On the other hand, when the video signal 106 is temporarily held, it isdifficult to hold data with an analog value; therefore, the video signal106 becomes a digital value. Thus, a pixel is controlled in a digitalmanner. Therefore, a digital gray scale method is to be used.

Thus, as a display mode, a case where the analog gray scale method isused is referred to as an analog mode, and a case where the digital grayscale method is used is referred to as a digital mode.

However, even in a case where the video signal 106 is temporarily held,it is also possible to hold signal with an analog value with the use ofa capacitor element or the like.

As described above, it is possible to switch between the analog grayscale method and the digital gray scale method by switching the displaymode control signal 107.

Next, FIG. 2 shows a case where part of the configuration in FIG. 1 isshown concretely. Note that although FIG. 2 is a case of two rows forsimplification, the present invention is not limited thereto. The numberof rows can be increased arbitrarily.

In the analog digital switching circuit 104, sampling switches 201 and202 are controlled by the sampling pulse that is sequentially outputtedfrom the shift register 103. Then, mode selection switches 203 and 204are controlled by the display mode control signal 107. The modeselection switches 203 and 204 are turned on and off exclusively. Inother words, when one of the mode selection switches 203 and 204 isturned on, the other switch is turned off. Whether the video signal 106is inputted into the digital data processing circuit 105 or inputtedinto the pixel array 101 is controlled by this mode selection switch. Inthe case of FIG. 2, when the mode selection switch 203 is turned on, thevideo signal 106 is transmitted to the pixel array 101 through thesampling switch 201 and the mode selection switch 203. In the samemanner, when a mode selection switch 205 is turned on, the video signal106 is transmitted to the pixel array 101 through the sampling switch202 and the mode selection switch 205. On the other hand, when the modeselection switch 204 is turned on, the video signal 106 is transmittedto the digital data processing circuit 105 through the sampling switch201 and the mode selection switch 204. In the same manner, when a modeselection switch 206 is turned on, the video signal 106 is transmittedto the digital data processing circuit 105 through the sampling switch202 and the mode selection switch 206.

In the digital data processing circuit 105, the video signal is storedand outputted in a latch 1 circuit 207 or a latch 2 circuit 208. In thelatch 1 circuit 207, the video signal 106 is inputted and storeddepending on the sampling pulse that is sequentially outputted from theshift register 103. Then, after storing the video signal 106 for onerow, a latch signal 211 is turned on. Consequently, the video signal 106stored in the latch 1 circuit is transferred to the latch 2 circuit 208.By performing such an operation, line-sequential driving can beperformed. The video signal is output to the pixel array 101 from thelatch 2 circuit 208 through output control switches 209 and 210. Theoutput control switches 209 and 210 are controlled depending on thedisplay mode control signal 107. In other words, when it is desired toinput the output of the latch 2 circuit 208 into the pixel array 101,for example, the output control switches 209 and 210 are turned on inthe case of a digital mode. On the other hand, when it is desired not toinput the output of the latch 2 circuit 208 into the pixel array 101,for example, the output control switches 209 and 210 are turned off inthe case of an analog mode. Consequently, the video signal 106 isinputted into the pixel array 101 through the mode selection switches203 and 205.

Here, FIG. 3 shows an example of the latch 1 circuit 207 and the latch 2circuit 208, each of which is constituted of a clocked inverter or aninverter. Note that the present invention is not limited thereto.

Note that, in the case of using the digital data processing circuit 105as like in FIG. 3, the sampling switches 201 and 202 can be omitted.This is because, in the case of using the digital data processingcircuit 105 as like in FIG. 3, it is possible to prevent data fromentering into the digital data processing circuit 105 even without thesampling switches 201 and 202.

In the pixel array 101, pixels 220 are arranged in matrix. FIG. 4 showsan example of one pixel 220. A selecting transistor 404 is controlledusing a gate signal line 401. When the selecting transistor 404 isturned on, a video signal is inputted into a storage capacitor 405 froma source signal line 402. Thus, a driving transistor 406 is turned onand off depending on the video signal and current flows to an oppositeelectrode 408 from a power supply line 403 through a light-emittingelement 407.

The power supply line 403 in FIG. 4 is connected to a power supply line221 in FIG. 2. In addition, the opposite electrode 408 in FIG. 4 isconnected to an opposite electrode 222 in FIG. 2. The opposite electrode222 is connected to all pixels in many cases. However, the presentinvention is not limited thereto.

Note that a pixel configuration is not limited to that shown in FIG. 4.For example, a configuration which corrects variations in the drivingtransistor may be employed.

The pixel configurations which correct variations can be roughly dividedinto two types: one is that corrects variations in threshold voltage andthe other is that inputs current as a video signal.

FIG. 32 shows the pixel configuration which corrects variations inthreshold voltage. A threshold voltage of a driving transistor 3101 isstored in a capacitor element 3104 by controlling a switch 3106. Aswitch 3103 serves to initialize the gate potential of the drivingtransistor 3101. Then, a video signal is inputted from a source signalline 3111 through a switch 3102. The video signal is stored in acapacitor element 3105. A switch 3107 controls a conducting state and/ora non-conducting state between a source terminal of the transistor 3101and the power supply line 3116. The first scanning line 3113 controls anon/off state of the a switch 3102, the second scanning line 3114controls an on/off state of a switch 3103, and the third scanning line3115 controls an on/off state of a switch 3107.

In FIG. 32, a wiring 3112 for initializing the gate potential of thedriving transistor 3101 is necessary. FIG. 33 shows a configurationwhere the wiring 3112 is removed from FIG. 32. A gate of the drivingtransistor 3101 is connected to a drain of the driving transistor 3101through a switch 3203.

Note that there are various pixel configurations which correctvariations in threshold voltage, which are not limited to theconfigurations shown in FIGS. 32 and 33. By using the pixelconfiguration which corrects variations in threshold voltage asdescribed above, variations in current flowing to the light-emittingelement can be reduced. In particular, luminance can be equalized in ananalog mode. Therefore, the pixel configuration which correctsvariations in threshold voltage is more suitable.

Next, FIG. 34 shows the pixel configuration which inputs current as avideo signal. Current in accordance with a video signal is supplied to asource signal line 3311. Then, the current flows to a driving transistor3301 when a switch 3302 and a switch 3304 are turned on; accordingly, agate-source voltage is generated depending thereon. The gate-sourcevoltage is stored in a capacitor element 3305; thereafter, current issupplied to a light-emitting element from the power supply line 3316when the switch 3302 and the switch 3304 are turned off, and a switch3306 is turned on. The first scanning line 3313 controls an on/off stateof the switch 3302, the second scanning line 3314 controls an on/offstate of the switch 3304, and the third scanning line 3315 controls anon/off state of a switch 3306. Note that a transistor to which signalcurrent is supplied is identical with a transistor for supplying currentto a light-emitting element in FIG. 34; however, they may be differentfrom each other. FIG. 35 shows this case. A transistor 3401 to whichsignal current is supplied and a transistor 3421 for supplying currentto a light-emitting element are different.

Note that there are various types of pixel configuration which correctvariations by inputting current, which is not limited to theconfigurations shown in FIGS. 34 and 35. By using the pixelconfiguration which corrects variations by inputting current asdescribed above, variations in current flowing to the light-emittingelement can be reduced. In particular, luminance can be equalized in ananalog mode. Therefore, the pixel configuration which correctsvariations by inputting current is more suitable.

Note that a light-emitting element arranged in a pixel is not limited toa specific one. As an example of the display element arranged in apixel, a display medium in which contrast varies by an electromagneticaction can be applied, such as an EL element (such as an organic ELelement, an inorganic EL element, or an EL element containing an organicmaterial and an inorganic material), an electron-emitting element, aliquid crystal element, electronic ink, an optical diffraction element,a discharging element, a digital micromirror device (DMD), apiezoelectric element, or a carbon nanotube. Note that an EL display isused as an EL panel type display device using the EL element, a fieldemission display (FED), an SED (Surface-conduction Electron-emitterDisplay) type flat display, or the like is used as a display deviceusing the electron-emitting element, a liquid crystal display is used asa liquid crystal panel type display device using the liquid crystalelement, an electronic paper is used as a digital paper type displaydevice using the electronic ink, a grating light valve (GLV) typedisplay is used as a display device using the optical diffractionelement, a plasma display is used as a PDP (Plasma Display Panel) typedisplay using the discharging element, a digital light processing (DLP)type display device is used as a DMD panel type display device using thedigital micromirror device, a piezoelectric ceramic display is used as adisplay device using the piezoelectric element, or a nano emissivedisplay (NED) is used as a display device using the carbon nanotube.

Note that the storage capacitor 405 serves to hold the gate potential ofthe driving transistor 406. Therefore, the storage capacitor 405 isconnected between a gate of the driving transistor 406 and the powersupply line 403; however, the present invention is not limited thereto.The storage capacitor 405 may be disposed so as to be able to store thegate potential of the driving transistor 406. In a case where the gatepotential of the driving transistor 406 can be held using the gatecapacitance of the driving transistor 406 or the like, the storagecapacitor 405 may be omitted.

Note that, as for the switches shown in FIG. 1 or the like, for example,the sampling switch 201, the mode selection switch 203, the outputcontrol switch 209, or the like, there is an electrical switch, amechanical switch, or the like. The switches are not particularlylimited as long as current flow can be controlled and various switchescan be used. For example, the switches may be a transistor, a diode (aPN diode, a PIN diode, a Schottky diode, a transistor connected as adiode, or the like) or a logic circuit that is a combination thereof.Thus, in a case of using a transistor as the switch, the transistoroperates as a mere switch; therefore, the polarity (conductivity type)of the transistor is not particularly limited. However, in a case wherelower off-current is desired, it is desirable to use a transistor havinga polarity with lower off-current. As the transistor with lowoff-current, a transistor provided with an LDD region, a transistorhaving a multi-gate structure, or the like can be used. In addition, itis desirable to use an N-channel transistor when a transistor to beoperated as a switch operates in a state where potential of a sourceterminal thereof is close to a lower potential side power supply (suchas Vss, GND, or 0 V), whereas it is desirable to use a P-channeltransistor when a transistor operates in a state where potential of asource terminal thereof is close to a higher potential side power supply(such as Vdd). This is because the absolute value of a gate-sourcevoltage can be increased, and the transistor easily operates as aswitch. Note that the switch may be of a CMOS type using both theN-channel transistor and the P-channel transistor. When the CMOS-typeswitch is employed, voltage outputted through the switch (that is,voltage inputted into the switch) may be high or low with respect to theoutputted voltage and the switch can be operated appropriately even whenthe situation is changed.

FIGS. 5A to 5D each shows an example of the switch. FIG. 5Aschematically shows a switch. FIG. 5B shows a switch using an ANDcircuit. Whether or not a signal of an input 501 is transmitted to anoutput 503 is controlled using a control line 502. In the case of FIG.5B, control can be performed in such a way that the output 503 is an Lsignal regardless of the input signal. However, the output 503 is neverin a floating state. Thus, the switch in FIG. 5B is preferably used suchas when the output 503 is connected to an input of a digital circuit. Inthe case of the digital circuit, an output is not put in a floatingstate even when an input is put in a floating state. When the input isput in a floating state, the output becomes unstable, which is notdesirable. Therefore, it is preferable to use the switch in FIG. 5B suchas when the output is connected to the input of a digital circuit.

Note that the switch in FIG. 5B is formed using the AND circuit;however, the present invention is not limited thereto. A similarfunction can also be performed when an OR circuit, a NAND circuit, or aNOR circuit is used.

On the other hand, a switch in FIG. 5C or 5D may be used when it isdesired to put the input in a floating state. The switch in FIG. 5C is acircuit referred to as a transmission gate, an analog switch, or thelike. The switch in FIG. 5C transmits the potential of an input 511 toan output 513 almost without change. Thus, the switch is preferable foranalog signal transmission. The switch in FIG. 5D is a circuit referredto as a clocked inverter or the like. The switch in FIG. 5D inverts andtransmits a signal of an input 521 to an output 523. Thus, the switch ispreferable for digital signal transmission.

According to the above, the switch in FIG. 5C is preferably used as thesampling switch 201, the mode selection switch 203 that transmit ananalog signal, or the like. Since the mode selection switch 204 thattransmit a digital signal is connected to the input of the latch 1circuit 207, which is a digital circuit, the switch in FIG. 5B ispreferably used. Since the output of the output control switch 209 orthe like needs to be put in a floating state, the switch in FIG. 5C or5D is preferable for the output control switch 209. However, it is adigital signal that is inputted into the output control switch 209;therefore, the switch in FIG. 5D is far preferable.

Accordingly, the selection of a display mode or the number of grayscales to be displayed can be controlled depending on external lightintensity. In this manner, it is possible to provide a display devicesuperior in visibility by controlling the gray scale number of a displayimage depending on peripheral illuminance. In other words, it ispossible to obtain a display device in which visibility is ensured in awide range of a dark place or under indoor florescent light to outdoorsun light.

Embodiment Mode 2

In this embodiment mode, a method for driving a pixel in an analog modewill be described.

FIGS. 6A and 6B show the relationship between a voltage and a currentapplied to a driving transistor and a light-emitting element. FIG. 6Ashows a circuit of a driving transistor 601 and a light-emitting element602. The driving transistor 601 and the light-emitting element 602 areconnected serially between a wiring 603 and a wiring 604. Since thewiring 603 has a higher potential than that of the wiring 604, a currentflows to the light-emitting element 602 from the driving transistor 601.

The driving transistor 406 in FIG. 4 corresponds to the drivingtransistor 601 in FIG. 6A, and the light-emitting element 407 in FIG. 4corresponds to the light-emitting element 602 in FIG. 6A.

FIG. 6B shows the relationship between a gate-source voltage (or theabsolute value thereof) of the driving transistor 60 and a currentflowing to the driving transistor 60 and the light-emitting element 602.As the gate-source voltage (or the absolute value thereof) is increased,a current value is also accordingly increased. This is because thedriving transistor 601 operates in a saturation region. In a saturationregion, a current value increases in proportion to the square of agate-source voltage of a transistor. As the gate-source voltage (or theabsolute value thereof) is further increased, a voltage applied to thelight-emitting element 602 is increased. Accordingly, a drain-sourcevoltage is decreased, and the driving transistor 601 operates in alinear region. As the drain-source voltage is decreased, the rate ofincrease in the current value is also decreased. Then, a current equalto or more than a certain current value does not flow.

In an analog mode, a gray scale is expressed using an analog gray scalemethod. Thus, by changing the gate-source voltage (or the absolute valuethereof) of the driving transistor 601 in an analog manner, the drivingtransistor 601 is desirably operated in such a state that the currentflowing to the driving transistor 601 and the light-emitting element 602also changes in an analog manner.

For example, as in a voltage range 620, the gate-source voltage (or theabsolute value thereof) of the driving transistor 601 may be controlledin such a state that the transistor operates in a saturation region froma state that almost no current flows. The state in which almost nocurrent flows corresponds to the case where the gate-source voltage ofthe driving transistor 601 is approximately equal to the thresholdvoltage of the driving transistor 601.

Alternatively, as in a voltage region 621, the gate-source voltage (orthe absolute value thereof) is increased and controlled from a state inwhich the gate-source voltage (or the absolute value thereof) of thedriving transistor 601 is certainly lower than the threshold voltage ofthe driving transistor 601, and the gate-source voltage (or the absolutevalue thereof) of the driving transistor 601 may be controlled in such astate that the driving transistor 601 operates in a saturation region.By making the gate-source voltage of the driving transistor 601 in ablack state certainly lower than the threshold voltage of the drivingtransistor 601, a black state can be assured. For example, if thecurrent characteristic of the driving transistor 601 varies, thethreshold voltage also varies. Thus, even when one pixel is in a blackstate, another pixel may slightly emit light. As a result, a decrease incontrast is caused. Thus, in order to prevent that, it is preferable tooperate the driving transistor 601 in such a voltage range as 621.

Note that in the voltage range 620 and the voltage range 621, thedriving transistor 601 is operated in a saturation region even if thegate-source voltage (or the absolute value thereof) is increased.However, the present invention is not limited thereto. As in a voltagerange 622 and a voltage range 623, the driving transistor 601 may beoperated not only in a saturation region but also in a linear region. Bychanging the gate-source voltage (or the absolute value thereof) of thedriving transistor 601 in an analog manner, the driving transistor 601may be operated also in a linear region as long as it is within such arange that a current flowing to the driving transistor 601 and thelight-emitting element 602 also changes in an analog manner.

Next, a case of performing optimization depending on the color of thelight-emitting element 602 will be described. The luminance andnecessary current value of the light-emitting element 602 vary dependingon color. Thus, a color balance needs to be adjusted. In order for thatto be done, the gate-source voltage (or the absolute value thereof) ofthe driving transistor 601 is desirably changed depending on color.Alternatively, the current supply capacity of the driving transistor 601(for example, a transistor width or the like) is desirably changeddepending on color. Alternatively, the light-emitting area of thelight-emitting element 602 is desirably changed depending on color.Further, alternatively, some of these are desirably combined. Thisenables to adjust the color balance.

Note that the potential of the wiring 603 can be changed depending oncolor. However, there is the disadvantage that the voltage at which thedriving transistor 601 is turned off is also changed depending on color.Therefore, the potential of the wiring 603 may be equivalent in allcolors.

Note that the case where the driving transistor 601 is a P-channeltransistor is described; however, the present invention is not limitedthereto. It is easy for those skilled in the art to reverse thedirection of current flow by using an N-channel transistor. In addition,it is also easy for those skilled in the art to reverse the direction ofcurrent flow either in a case of a P-channel transistor or in a case ofan N-channel transistor. In this case, the amount of gate-source voltageis affected by the voltage-current characteristic of the light-emittingelement 602.

Note that this embodiment mode describes the pixel of Embodiment Mode 1in detail. Therefore, the content described in this embodiment mode canbe arbitrarily combined with the content described in Embodiment Mode 1.

Embodiment Mode 3

In this embodiment mode, a method for driving a pixel in a digital modewill be described.

The relationship between the gate-source voltage (or the absolute valuethereof) of the driving transistor 601 and a current flowing to thedriving transistor 601 and the light-emitting element 602, shown in FIG.6B, is referred to. In a digital mode, control is performed in binarylike on and off, or H and L. In other words, whether or not a currentflows to the light-emitting element 602 is controlled. First, a casewhere current does not flow is considered. In this case, the gate-sourcevoltage (or the absolute value thereof) of the driving transistor 601may be 0 V or more as indicated by a voltage 624, a voltage 625, and avoltage 626 and equal to or lower than the threshold voltage of thedriving transistor 601.

Next, a case where current flows is considered. In this case, thedriving transistor 601 may be operated in a saturation region, a linearregion, or a region where voltage is further increased and a currentvalue is not increased, or the like with the gate-source voltage (or theabsolute value thereof) of a voltage 627, a voltage 628, and a voltage629.

For example, operation in a saturation region has the advantage that thevalue of a current flowing through the light-emitting element 602 doesnot vary even when the voltage-current characteristic thereof isdeteriorated. Therefore, the current value is hardly affected byburn-in. However, when the current characteristic of the drivingtransistor 601 varies, the current flowing therethrough also varies.Thus, uneven display may be caused.

On the other hand, if the driving transistor 601 is operated in a linearregion, the value of a current flowing through the driving transistor601 is hardly affected even when the current characteristic of thedriving transistor 601 varies. Therefore, uneven display is hardlycaused. In addition, power consumption can be reduced because thegate-source voltage (or the absolute value thereof) of the drivingtransistor 601 does not become too high and the voltage between thewiring 603 and the wiring 604 does not need to be high.

Further, when the gate-source voltage (or the absolute value thereof) ofthe driving transistor 601 is high, the value of a current flowingthrough the driving transistor 601 is hardly affected even if thecurrent characteristic thereof varies. However, when the voltage-currentcharacteristic of the light-emitting element 602 is deteriorated, thevalue of a current flowing therethrough may vary. Therefore, the currentvalue is easily affected by burn-in.

As described above, when the driving transistor 601 is operated in asaturation region, the value of a current flowing therethrough does notvary even if the characteristics of the light-emitting element 602 vary.Therefore, in this case, it can be assumed that the driving transistor601 operates as a current source. Thus, such driving is referred to asconstant current driving.

When the driving transistor 601 is operated in a linear region, thecurrent value does not vary even if the current characteristic of thedriving transistor 601 varies. Therefore, in this case, it can beassumed that the driving transistor 601 operates as a switch.Accordingly, the voltage of the wiring 603 can be considered to beapplied to the light-emitting element 602 without any change. Thus, suchdriving is referred to be constant voltage driving.

In a digital mode, either constant voltage driving or constant currentdriving may be employed. However, constant voltage driving is preferablebecause variations in transistor does not affect the constant voltagedriving and can reduce power consumption.

Next, a case of performing optimization depending on the color of thelight emitting element 632 will be described. The case of constantcurrent drive is similar to an analog mode.

In the case of constant voltage drive, even if the gate-source voltage(or the absolute value thereof) of the driving transistor 601 and thecurrent supply capacity of the driving transistor 601 (for example, atransistor width or the like) are changed depending on color, the valueof a current flowing therethrough does not vary so much. This is becausethe driving transistor operates 601 as a switch.

Therefore, the light-emitting area of the light-emitting element 602 isdesirably changed depending on color. Alternatively, the potential ofthe wiring 603 can be changed depending on color. Alternatively, theseare desirably combined. This enables to adjust the color balance.

Note that this embodiment mode describes the pixel of Embodiment Mode 1in detail. Therefore, the content described in this embodiment mode canbe arbitrarily combined with the content described in Embodiment Modes 1and 2.

Embodiment Mode 4

In a case of a digital mode, only a binary of a light-emitting state anda non-light-emitting state can be expressed if nothing is done.Accordingly, another method may be used in combination to achievemultiple gray scales. A driving method of a pixel in the case wheremultiple gray scales are achieved will be described.

As a driving method for achieving multiple gray scales, there are a timegray scale method and an area gray scale method. The time gray scalemethod is a method for expressing a gray scale by changing the length oflight-emitting time during a certain period. The area gray scale methodis a method for expressing a gray scale by changing the size of alight-emitting area.

Note that the time gray scale method and the area gray scale method maybe combined with each other.

Herein, the time gray scale method will be described in detail. In adigital time gray scale method, one-frame period is divided into aplurality of sub-frame periods. Then, a gray scale is expressed bychanging the length of a lighting period during each sub-frame period.

FIG. 7 shows a timing chart in a case where a period where signals arewritten to a pixel and a period where light is emitted are separated.First, signals for one screen are inputted into all pixels in asignal-writing period. During this period, pixels emit no light. Afterthe signal-writing period, a light-emitting period starts and pixelsemit light. Next, a subsequent sub-frame starts and signals for onescreen are inputted into all pixels in a signal-writing period. Duringthis period, pixels emit no light. After the signal-writing period, alight emitting period starts and pixels emit light.

By repeating similar operations, a gray scale can be expressed. At thistime, it is possible to express various gray scales by having the powerof 2 for the length of the lighting period in each sub-frame period aslike 1:2:4:8: . . . .

A pixel configuration in this case may have a configuration of FIG. 4.

Note that, in a signal-writing period, potentials of a power supply line403 and an opposite electrode 408 are controlled so that no voltage isapplied to a light-emitting element 407. For example, the potential ofthe opposite electrode 408 is increased so that no voltage is applied tothe light-emitting element 407, or the opposite electrode 408 may bemade in a floating state without supplying an electric charge.Consequently, the light-emitting element 407 can be prevented fromemitting light in a signal-writing period.

Next, FIG. 8 shows a timing chart in a case where a period where asignal is written to a pixel and a period where light is emitted are notseparated. Immediately after a signal is written to each row, alight-emitting period starts.

In a certain row, after writing of signals and a predeterminedlight-emitting period are completed, a signal writing operation startsin a subsequent sub-frame. By repeating such operations, each length ofthe light-emitting periods can be controlled.

In this manner, many sub-frames can be arranged in one frame even ifsignals are written slowly. In addition, since the ratio of alight-emitting period during one-frame period (a so-called duty ratio)can be high, it is possible to reduce power consumption, suppressdeterioration of the light-emitting element, or suppress a pseudocontour.

A pixel configuration in this case may have a configuration of FIG. 4.In this case, where a time is t0 in FIG. 8, it is necessary to inputsignals into pixels of plural rows at the same time. Usually, it isimpossible to input signals into pixels of three rows at the same time.Thus, as shown in FIG. 9, one gate selection period is divided into aplurality of periods (three in FIG. 9). Each gate signal line 401 isselected in each of the divided selection periods and a correspondingsignal is inputted into a source signal line 402. For example, in onegate selection period, an i-th row is selected in G1(t0), a j-th row isselected in G2(t0), and a k-th row is selected in G3(t0). Accordingly,an operation can be performed as if the three rows were selected at thesame time in the one gate selection period.

Note that, although FIGS. 8 and 9 each show the case where signals areinputted into pixels of three rows at the same time, the presentinvention is not limited thereto. A signal may also be inputted intomore rows or few rows.

Note that details of such a driving method are disclosed in, forexample, Japanese Patent Application Laid-Open No. 2001-324958 and thelike, which can be applied in combination with the present invention.

Then, FIG. 10 shows a timing chart in a case where signals in pixels areerased. In each row, a signal writing operation is performed and thesignals in the pixels are erased before a subsequent signal writingoperation. According to this, the length of a light-emitting period canbe easily controlled.

In a certain row, after writing of signals and a predetermined lightemitting period are completed, a signal writing operation starts in asubsequent sub-frame. In a case where a light-emitting period is short,a signal erasing operation is performed to provide a non-light-emittingstate. By repeating such operations, the lengths of the light-emittingperiods can be controlled.

According to this, many sub-frames can be arranged in one frame even ifsignals are written slowly. Further, in the case of performing thesignal erasing operation, data for erasing is not required to beobtained as well as a video signal; therefore, the driving frequency ofa source driver can also be reduced.

FIG. 11 shows a pixel configuration in this case. An erasing transistor1104 is connected between a gate of a driving transistor 406 and thepower supply line 403.

A selecting transistor 404 is controlled using the gate signal line 401.When the selecting transistor 404 is turned on, a video signal isinputted into a storage capacitor 405 from the source signal line 402.Thus, a driving transistor 406 is turned on and off depending on thevideo signal and current flows to the opposite electrode 408 through thepower supply line 403 to the light-emitting element 407.

When it is desired to erase a signal, a second gate line 1101 isselected to turn the erasing transistor 1104 on, so that the drivingtransistor 406 is turned off. Then, no current flows from the powersupply line 403 to the opposite electrode 408 through the light-emittingelement 407. Consequently, a non-light-emitting period can be providedand the length of a light-emitting period can be freely controlled.

Although the erasing transistor 1104 is used in FIG. 11, another methodcan also be used. This is because a non-light-emitting period mayforcibly be provided so that no current is supplied to thelight-emitting element 407. Thus, a non-light-emitting period may beprovided by arranging a switch somewhere in a path where a current flowsfrom the power supply line 403 to the opposite electrode 408 through thelight-emitting element 2104 and controlling on/off of the switch.Alternatively, a gate-source voltage of the driving transistor 406 maybe controlled to forcibly turn the driving transistor off.

FIG. 12 shows an example of a pixel configuration in the case where thedriving transistor is forcibly turned off. An erasing diode 1204 isconnected between the gate of the driving transistor 406 and a secondgate line 1201.

When it is desired to erase a signal, the second gate line 1201 isselected (supplied with a high potential here) to turn the erasing diode1204 on, so that current flows from the second gate line 1201 to thegate of the driving transistor 406. Consequently, the driving transistor2203 is turned off. Then, no current flows from the power supply line403 to the opposite electrode 408 through the light-emitting element407. Consequently, a non-light-emitting period can be provided and thelength of a light-emitting period can be freely controlled.

When it is desired to hold a signal, the second gate line 1201 is notselected (supplied with a low potential here). Then, the erasing diode1204 is turned off and the gate potential of the driving transistor 406is thus held.

Note that the erasing diode 1204 may be any element as far as it has arectifying property. The erasing diode 1204 may be a PN diode, a PINdiode, a Schottky diode, or a zener diode.

In addition, a diode-connected transistor (a gate and a drain thereofare connected) may be used as well. FIG. 23 shows a circuit diagram inthis case. As the erasing diode 1204, a diode-connected transistor 1304is used. Although an N-channel transistor is used here, the presentinvention is not limited thereto and a P-channel transistor may also beused.

Note that a driving method as shown in FIG. 10 can be achieved using thecircuit in FIG. 4 as still another circuit. FIG. 9 shows a timing chartof this case. As shown in FIG. 9, one gate selection period is dividedinto three; however, here, one gate selection period is divided intotwo. Each gate line is selected in each of the divided selection periodsand a corresponding signal (a video signal and an erasing signal) isinputted into the source signal line 402. For example, in one gateselection period, the i-th row is selected in the first half of theperiod and the j-th row is selected in the latter half of the period.Then, when the i-th row is selected, a video signal for it is inputted.On the other hand, when the j-th row is selected, a signal for turningthe driving transistor off is inputted. Accordingly, an operation can beperformed as if the two rows are selected at the same time in the onegate selection period.

Note that details of such a driving method are disclosed in, forexample, Japanese Patent Application Laid-Open No. 2001-324958 and thelike, which can be applied in combination with the present invention.

Note that the timing charts, pixel configurations, and driving methodsthat are shown in this embodiment mode are examples, and the presentinvention is not limited thereto. It is possible to apply various timingcharts, pixel configurations, and driving methods.

Note that this embodiment mode describes the pixel of Embodiment Modes 1to 3 in detail. Therefore, the content described in this embodiment modecan be arbitrarily combined with the content described in EmbodimentModes 1 to 3.

Embodiment Mode 5

In the case of a digital mode, only a binary of light emission andnon-light emission can be expressed if nothing is done. However, powerconsumption can be reduced considerably by performing display only withthe binary.

In addition, since a gray scale can be distanced more clearly, it ispossible to improve visibility of a display screen of a display panel.

Such a display mode is to be referred to as a binary mode.

Herein, performing display with the binary mode will be described indetail. As for the pixel configuration, various configurations describedin Embodiment Mode 4 may be used.

First, FIG. 14A shows a timing chart in a case where signals are writtenwhile scanning rows with almost the same rate as each driving methoddescribed in Embodiment Mode 4. In this case, since a source driver or agate driver is to be operated with almost the same frequency, signalsthat are inputted into these drivers (for example, a clock signal or thelike) may have the same frequency. Therefore, a frequency generatingcircuit is not necessarily plural, which may be one. Thus, the circuitcan be reduced in size and the cost reduction can be realized.

However, in this case, the duty ratio of the binary mode may beincreased compared with an analog mode or a digital mode other than thebinary mode. Thus, in order to reduce the duty ratio, an erasingoperation may also be performed. FIG. 14B shows a timing chart in thiscase.

Next, FIG. 15A shows a timing chart in a case where signals are writtenwhile scanning rows with rate lower than the case of each driving methoddescribed in Embodiment Mode 4. Herein, all rows are scanned takingone-frame period. In this case, a source driver or a gate driver is tobe operated with the low frequency. Thus, a signal that is supplied tothe source driver or the gate driver or voltage of a power supply may below. Therefore, power consumption can be reduced.

However, also in this case, the duty ratio of the binary mode may beincreased compared with an analog mode or a digital mode other than thebinary mode. Thus, in order to reduce the duty ratio, an erasingoperation may also be performed. FIG. 15B shows a timing chart in thiscase.

Note that, also in the analog mode, an operation may be performedaccording to the timing chart of FIGS. 14A and 14B or FIGS. 15A and 15B.

Note that, in a case of performing color display with the binary mode,display is performed with binary in each of R, G, and B; thus, it ispossible to display eight colors in total.

Note that, although the case of displaying only with the binary isdescribed here, the present invention is not limited thereto. Not onlythe binary, that is, display only with one bit, but also display withtwo bits or more may be performed. For example, FIGS. 16A and 16B show acase of performing three-bit display. As shown in FIGS. 14A and 14B orFIGS. 15A and 15B, an erasing operation may be performed, or rate forscanning rows may be slowed down.

Note that this embodiment mode describes the pixel of Embodiment Modes 1to 4 in detail. Therefore, the content described in this embodiment modecan be arbitrarily combined with the content described in EmbodimentModes 1 to 4.

Embodiment Mode 6

Thus far, various display modes are described. First, there are ananalog mode and a digital mode, and the digital mode has a normal modeand a binary mode.

Note that, the digital mode having fewer display gray scales than thenormal mode and having more display gray scales than the binary mode isto be referred to as a multi-valued mode. In other words, the normalmode refers to a display mode in a case where the gray scale number arethe most in the digital mode, and the normal mode is hereinafterreferred to as a full gray scale mode. The multi-valued mode can reducepower consumption than the full gray scale mode and an image can bedisplayed more clearly than the binary mode.

Thus, the analog mode, the digital mode in the full gray scale mode, thedigital mode in the multi-valued mode, or the digital mode in the binarymode may be switched depending on illuminance of an image to bedisplayed, outside, or a periphery.

For example, when it is desired to display a clear image like aphotograph or the like or when peripheral illuminance is not so high, ananalog mode or a full gray scale mode is used for display. Accordingly,it is possible to express a gray scale precisely and finely. In thiscase, a display mode may be switched depending on a signal that isinputted as a video signal 106. For example, when the video signal 106is an analog signal, an analog mode may be used, and when the videosignal 106 is a digital signal, a full gray scale mode may be used. Asfor the full gray scale mode, it is desirable to perform display of sixbits or more, more desirably, display of eight bit or more. As for theanalog mode, it is desirable to perform display of eight bits or more.

In addition, in a case of displaying a character mainly, for example, ina case of reading an e-mail, reading an electronic book, or whenilluminance is as high as outdoor, it is desirable to perform display byusing a digital mode in a binary mode. Accordingly, power consumptioncan be reduced. In addition, since a gray scale can be distinguishedmore clearly, it is possible to improve visibility of a display screenof a display panel.

Moreover, for example, when it is desired to express a gray scale of anillustration, an animation, a cartoon, or the like without necessity toexpress finely as a photograph or the like, or in a case of illuminanceas like clouded outdoor, it is desirable to perform display by using amulti-valued mode. Accordingly, power consumption can be reduced and aclear image can be displayed.

Note that this embodiment mode describes the pixel of Embodiment Modes 1to 5 in detail. Therefore, the content described in this embodiment modecan be arbitrarily combined with the content described in EmbodimentModes 1 to 5.

Embodiment Mode 7

Next, as for each display mode, that is, an analog mode, a full grayscale mode, a digital mode in a multi-valued mode, a digital mode in abinary mode, or the like, potential of a power supply line 403 and anopposite electrode 408 in each display mode will be described.

First, a case where potential of the power supply line 403 is changedwill be described. FIG. 17 shows a case where a display mode is changedin the order of an analog mode, a digital mode of a binary mode, and adigital mode of a full gray scale mode. In the analog mode, a drivingtransistor is mainly operated in a saturation region; therefore, it isnecessary to increase the voltage between the power supply line 403 andthe opposite electrode 408. In other words, it is necessary to increasethe potential of the power supply line 403. In the binary mode, adriving transistor is mainly operated in a linear region; therefore, thevoltage between the power supply line 403 and the opposite electrode 408is reduced. In other words, the potential of the power supply line 403may be low. In addition, since the binary mode has a high duty ratio insome cases, with the consideration thereof, the voltage between thepower supply line 403 and the opposite electrode 408 is reduced. In thedigital mode in the full gray scale mode, since the duty ratio getshigher than that of the binary mode in some cases, in this case, thevoltage between the power supply line 403 and the opposite electrode 408gets higher than that of the binary mode. In other words, the potentialof the power supply line 403 is increased. However, a driving transistoris mainly operated in a linear region; therefore, the voltage may belower than that of the analog mode. In other words, the potential of thepower supply line 403 may be low.

Note that, since a characteristic of a light-emitting element differs ineach color, in the binary mode or the full gray scale mode, thepotential of the power supply line 403 may differ in each color.

Although the potential of the power supply line 403 is changed dependingon a display mode in FIG. 17, the present invention is not limitedthereto. The potential of the opposite electrode 408 may be changed.FIG. 18 shows this case.

In the analog mode, a driving transistor is mainly operated in asaturation region; therefore, it is necessary to increase the voltagebetween the power supply line 403 and the opposite electrode 408. Inother words, it is necessary to reduce the potential of the oppositeelectrode 408. In the binary mode, a driving transistor is mainlyoperated in a linear region; therefore, the voltage between the powersupply line 403 and the opposite electrode 408 is reduced. In otherwords, the potential of the opposite electrode 408 may be high. Inaddition, since the binary mode has a high duty ratio in some cases,with the consideration thereof, the voltage between the power supplyline 403 and the opposite electrode 408 is reduced. In the digital modein the full gray scale mode, since the duty ratio gets higher than thatof the binary mode in some cases, in this case, the voltage between thepower supply line 403 and the opposite electrode 408 gets higher thanthat of the binary mode. In other words, the potential of the oppositeelectrode 408 is reduced. However, a driving transistor is mainlyoperated in a linear region; therefore, the voltage may be lower thanthat of the analog mode. In other words, the potential of the oppositeelectrode 408 may be high.

Note that, since a characteristic of a light-emitting element differs ineach color, in the binary mode or the full gray scale mode, thepotential of the opposite electrode 403 may differ in each color.

FIGS. 17 and 18 may be combined. In other words, both potentials of thepower supply line 403 and the opposite electrode 408 may be changeddepending on a display mode. However, the potential of the power supplyline 403 and the opposite electrode 408 are not necessarily changed evenwhen display modes are changed.

Note that, although FIGS. 17 and 18 show a case of changing displaymodes in the order of an analog mode, a digital mode in a binary mode,and a digital mode in a full gray scale mode, the present invention isnot limited thereto. The display mode may be changed in any order. Inaddition, FIGS. 19 and 20 each shows a case of using a multi-valuedmode. It is desirable that the potential of the power supply line 403 inthe multi-valued mode be lower than that of the full gray scale mode andhigher than that of the binary mode. Moreover, it is desirable that thepotential of the opposite electrode 408 in the multi-valued mode behigher than that of the full gray scale mode and lower than that of thebinary mode.

By using these modes, it is possible to display a clear image whilereducing power consumption depending on an image to be displayed.

Then, a circuit configuration in a case where the potential of the powersupply line 403 and the opposite electrode 408 is changed will bedescribed. FIG. 21 shows a configuration diagram when a plurality ofpower supplies is used. A display mode is determined in a display modeswitching control circuit 2101. Then, a display mode control signal 107is outputted to control. A wiring 2102 is connected to power supplylines 403 and 221, the opposite electrode 408, an opposite electrode222, or the like. Then, it is controlled that which of voltage frompower supplies 2105 and 2106 is outputted by using a switch 2103 or2104. In FIG. 21, although the case of the two power supplies 2105 and2106 and the two switches 2103 and 2104 is shown, the present inventionis not limited thereto. Therefore, for example, a variable voltage 2205may be used and disposed as like in FIG. 22.

Note that this embodiment mode describes the pixel of Embodiment Modes 1to 6 in detail. Therefore, the content described in this embodiment modecan be arbitrarily combined with the content described in EmbodimentModes 1 to 6.

Embodiment Mode 8

Next, as for each display mode, that is, an analog mode and a digitalmode, potential of a video signal will be described.

As for a driving transistor 406 in FIG. 4 or a driving transistor inFIG. 6A, the potential of a signal that is inputted into a gateelectrode will be described.

First, in the analog mode, a driving transistor is mainly operated in asaturation region; therefore, a gate-source voltage of the drivingtransistor (an absolute value thereof) is low. On the other hand, in thedigital mode, a driving transistor is mainly operated in a linearregion; therefore, a gate-source voltage of the driving transistor (anabsolute value thereof) is high.

Thus, a video signal to be inputted into a pixel may be differed in ananalog mode and a digital mode. Thus, a circuit for controlling a levelmay be arranged. FIG. 23 shows a circuit diagram in this case. A levelcontrol circuit 2301 may be arranged after a latch 2 circuit. In thiscircuit, a video signal to be inputted into a pixel may be differeddepending on a display mode.

FIG. 24 shows a configuration of the level control circuit 2301. It iscontrolled whether the level is converted or not by switching to pass ornot to pass a level shifter 2401.

Note that this embodiment mode describes the pixel of Embodiment Modes 1to 7 in detail. Therefore, the content described in this embodiment modecan be arbitrarily combined with the content described in EmbodimentModes 1 to 7.

Embodiment Mode 9

Next, description is made on the layout of a pixel in the display deviceof the present invention. As an example, FIG. 25 is a layout diagram ofthe circuit diagram shown in FIG. 4. Note that the circuit diagram andthe layout diagram are not limited to FIGS. 4 and 25.

An electrode 407A of a selection transistor 404, a driving transistor406, and a light-emitting element 407 are arranged. A source and a drainof the selection transistor 404 are connected to a source signal line402 and a gate of the driving transistor 406, respectively. A gate ofthe selection transistor 404 is connected to the gate signal line 401. Asource and a drain of the driving transistor 406 are connected to apower supply line 403 and an electrode of the light-emitting element407, respectively. A storage capacitor 405 is connected between the gateof the driving transistor 406 and the power supply line 403.

The source signal line 402 and the power supply line 403 are formed witha second wiring, and the gate signal line 401 is formed with a firstwiring.

In a case of a top-gate structure, a substrate, a semiconductor layer, agate insulating film, the first wiring, an interlayer insulating film,and the second wiring are sequentially formed. In a case of abottom-gate structure, a substrate, the first wiring, a gate insulatingfilm, a semiconductor layer, an interlayer insulating film, and thesecond wiring are sequentially formed.

Next, FIG. 36 shows a cross-sectional view of a pixel constituted by athin film transistor (TFT) and a light-emitting element connectedthereto.

In FIG. 36, a semiconductor layer 702 constituting a base layer 701 anda TFT 750 and a semiconductor layer 712 constituting one of electrodesof a capacitance portion 751 are formed over a substrate 700. Over theselayers, a first insulating layer 703 is formed, which functions as agate insulating layer for the TFT 750, and as a dielectric layer formingcapacitance for the capacitance portion 751.

Over the first insulating layer 703, a gate electrode 704 and aconductive layer 754 forming the other electrode of the capacitanceportion 751 are formed. A wiring 707 connected to the TFT 750 isconnected to a first electrode 708 of a light-emitting element 712. Thefirst electrode 708 is formed over a third insulating layer 706. Asecond insulating layer 705 may be formed between the first insulatinglayer 703 and the third insulating layer 706. The light-emitting element712 is constituted by the first electrode 708, an EL layer 709, and asecond electrode 710. In addition, a fourth insulating layer 711 isformed so as to cover a peripheral edge of the first electrode 708 and aconnection portion of the first electrode 708 and a wiring 707.

Then, the details of the structure shown above will be explained. As thesubstrate 700, a glass substrate such as a barium borosilicate glass andan alumino borosilicate glass, a quartz substrate, a ceramic substrate,or the like can be used. In addition, a metal substrate includingstainless-steel or a semiconductor substrate with an insulating filmformed over the surface may be used. A substrate formed of a syntheticresin having flexibility such as a plastic may be used. The surface ofthe substrate 700 may be planarized by polishing such as chemicalmechanical polishing (CMP).

As the base layer 701, an insulating film such as silicon oxide, siliconnitride, or silicon nitride oxide can be used. It is possible to preventan alkali metal such as Na and an alkaline earth metal contained in thesubstrate 700 from diffusing into the semiconductor layer 702 and havingnegative effects on the characteristics of the TFT 750 by the base layer701. Although FIG. 36 has a single layer structure, the base layer 701may be formed in two layers or in a multilayer of two or more layers.Note that, in a case where a substrate having no problem of impuritydiffusion, such as a quartz substrate, is used, the base layer 701 isnot necessarily provided.

In addition, a surface of the glass substrate may be processed directlyby dense plasma the electron temperature of which is 2 eV or less, theion energy is 5 eV or less, and the electron density is approximatelyfrom 1×10¹¹ to 5×10¹³/cm³ that is excited by a microwave. For generatingplasma, a plasma treatment apparatus of microwave excitation using aradial slot antenna can be used. At this time, when nitrogen (N₂), or anitride gas such as ammonia (NH₃) or nitrous oxide (N₂O) is introduced,the surface of the glass substrate can be nitrided. Since a nitridelayer formed over the surface of the glass substrate contains siliconnitride as its main component, the nitride layer can be used a blockinglayer of an impurity diffused from the glass substrate side. A siliconoxide film or a silicon oxynitride film may be formed over the nitridelayer by a plasma CVD method so as to be the base layer 701.

Besides, by performing the same plasma treatment to a surface of thebase layer 701 of silicon oxide, silicon oxynitride, or the like, thesurface and 1 to 10 nm deep from the surface can be treated fornitriding. By this extremely thin layer of silicon nitride, a blockinglayer, which does not have an effect of stress on a semiconductor layerformed thereover, can be made.

As the semiconductor layer 702 and the semiconductor layer 752, it ispreferable to use patterned crystalline semiconductor films. Note thatpatterning refers to processing the shape of a film, that is, forming afilm pattern by a photolithography technique (for example, forming acontact hole in photosensitive acrylic or processing the shape ofphotosensitive acrylic so as to be a spacer is included), or forming amask pattern by a photolithography technique to perform etchingprocessing by using the mask pattern. A crystalline semiconductor filmcan be obtained by crystallizing an amorphous semiconductor film. As amethod for crystallization, a laser crystallization method, a thermalcrystallization method using an RTA or an annealing furnace, a thermalcrystallization method using a metal element promoting crystallization,or the like can be used. The semiconductor layer 702 has a channelforming region and a pair of impurity regions to which an impurityelement imparting one conductivity type is added. Note that an impurityregion to which the impurity element is added at low concentration maybe provided between the channel forming region and the pair of impurityregions. The semiconductor layer 752 can have a structure in which animpurity element imparting one conductivity type or an oppositeconductivity type is entirely added.

Silicon oxide, silicon nitride, silicon nitride oxide, or the like canbe used as the first insulating layer 703, which can be formed in asingle layer or in a stacked layer of a plurality of films. In thiscase, in the same manner as the above case, oxidation or nitriding maybe performed to a surface of the insulating film to have a dense surfaceby a high-density plasma treatment an electron temperature of which is 2eV or less, an ion energy is 5 eV or less, and an electron density isapproximately 1×10¹¹ to 5×10¹³/cm³, that is excited by a microwave. Thistreatment may be performed before forming the first insulating layer703. In other words, plasma treatment is performed to a surface of thesemiconductor layer 702. At this time, by setting a substratetemperature at 300 to 450° C. and performing the treatment in anoxidative atmosphere (O₂, N₂O, or the like) or in a nitriding atmosphere(N₂, NH₃, or the like), a preferable interface with the gate insulatinglayer to be deposited thereover can be formed.

As the gate electrode 704 and the conductive layer 754, a single layeror stacked layer structure formed of an alloy or a compound includingone or more elements of Ta, W, Ti, Mo, Al, Cu, Cr and Nd can be used.

The TFT 750 includes the semiconductor layer 702, the gate electrode704, and the first insulating layer 703 between the semiconductor layer702 and the gate electrode 704. In FIG. 36, as the TFT 750 configuring apixel, a TFT which is connected to the first electrode 708 of thelight-emitting element 712 is shown. This TFT 750 has a structure of amulti-gate type in which a plurality of gate electrodes 704 are placedover the semiconductor layer 702. In other words, the TFT 750 has astructure in which a plurality of TFT are connected in series. Accordingto such a structure, the unconsidered increase of off current can besuppressed. Note that, although FIG. 36 shows the TFT 750 as a top-gateTFT, the TFT 750 may be a bottom-gate TFT having a gate electrode undera semiconductor layer, or a dual-gate TFT having gate electrodes aboveand under a semiconductor layer.

A capacitor portion 751 includes the first insulating film 703 as adielectric, and the semiconductor layer 712 and the conductive layer 754opposing to each other by sandwiching the first insulating film 703, asa pair of electrodes. Note that FIG. 36 shows an example where one ofthe pair of electrodes is the semiconductor layer 752 which is formedsimultaneously with the semiconductor layer 702 of the TFT 750 and theother conductive layer 754 is a layer formed simultaneously with thegate electrode 704; however, the present invention is not limitedthereto.

It is desirable that the second insulating layer 705 be an insulatingfilm with a barrier property blocking an ionic impurity, such as asilicon nitride film. The second insulating layer 705 is formed ofsilicon nitride or silicon oxynitride. The second insulating layer 705includes a function as a protective film preventing contamination of thesemiconductor layer 702. After depositing the second insulating layer705, high-density plasma treatment exited by a microwave as describedabove may be performed, introducing a hydrogen gas, so that the secondinsulating layer 705 is hydrogenated. Alternatively, the secondinsulating layer 705 may be nitrided and hydrogenated, by introducing anammonia gas. Alternatively, oxynitride treatment and hydrogen treatmentmay be performed, by introducing oxygen, an N₂O gas or the like, and ahydrogen gas. By performing nitriding, oxidation, or oxynitridetreatment by this method, a surface of the second insulating layer 705can become dense. Accordingly, a function as a protective film can bestrengthened. As for hydrogen introduced to the second insulating layer705, by performing heat treatment of 400 to 450° C., hydrogen isreleased from silicon nitride forming the second insulating layer 705,and hydrogenation of the semiconductor layer 702 can be performed.

As the third insulating layer 706, an inorganic insulating film or anorganic insulating film can be used. As the inorganic insulating film, asilicon oxide film formed by a CVD method, an SOG (Spin On Glass) film(coated silicon oxide film), or the like can be used. As the organicinsulating film, a film of polyimide, polyamide, BCB (benzocyclobutene),acrylic, positive type photosensitive organic resin or negative typephotosensitive organic resin, or the like can be used. In addition, asthe third insulating layer 706, a material of which the skeletonstructure is constituted by the bond of silicon (Si) and oxygen (O) canbe used. As a substituent of this material, an organic group includingat least hydrogen (an alkyl group and aromatic hydrocarbon, for example)is used. A fluoro group may also be used as the substituent.Alternatively, an organic group including at least hydrogen and a fluorogroup may also be used as the substituent.

As the wiring 707, a single layer or a stacked layer structure formed ofan alloy containing one or plural kinds of elements of Al, Ni, C, W, Mo,Ti, Pt, Cu, Ta, Au and Mn can be used.

Either one or both of the first electrode 708 and the second electrode710 can be made as transparent electrodes. As the transparent electrode,indium oxide containing tungsten oxide (IWO), indium oxide containingtungsten oxide and zinc oxide (IWZO), indium oxide containing titaniumoxide (ITiO), indium tin oxide containing titanium oxide (ITTiO), indiumtin oxide containing molybdenum, or the like can be used. Of course,indium tin oxide (ITO), indium zinc oxide (IZO), indium tin oxide withsilicon oxide added (ITSO), or the like can also be used.

At least one of the first electrode 708 and the second electrode 710 maybe formed of a material having no transparency. For example, an alkalimetal such as Li and Cs, an alkali earth metal such as Mg, Ca and Sr, analloy containing these (Mg:Ag, Al:Li, Mg:In or the like), an compoundthereof (CaF₂, Ca₃N₂), or a rare-earth metal such as Yb and Er can beused.

The fourth insulating layer 711 can be formed using the same material asthe third insulating layer 706.

The light-emitting element 712 is constituted by the EL layer 709, thefirst electrode 708 and the second electrode 710 sandwiching the ELlayer 709. One of the first electrode 708 and the second electrode 710corresponds to an anode, and the other corresponds to a cathode. When avoltage larger than a threshold voltage is applied with a forward biasbetween the anode and the cathode, current flows from the anode to thecathode and the light-emitting element 712 emits light.

The EL layer 709 is constituted by a single layer or a plurality oflayers. In the case where it is constituted by a plurality of layers,the layers can be classified into a hole injecting layer, a holetransporting layer, a light-emitting layer, an electron transportinglayer, an electron injecting layer, and the like, depending on thecarrier transport property. Note that the boundary between each layer isnot necessarily clear, and there is a case where the interface isunclear because materials constituting each layer are mixed partly. Anorganic material and an inorganic material can be used for each layer.Any material of a high molecular compound, a middle molecular compound,and a low molecular compound can be used as the organic material.

It is preferable that the EL layer 709 be constituted using a pluralityof layers having different functions, such as a hole injecting andtransporting layer, a light-emitting layer, and an electron injectingand transporting layer. It is preferable that the hole injecting andtransporting layer be formed of composite materials containing anorganic compound material having a hole transporting property and aninorganic compound material showing an electron-accepting property tothe organic compound material. Due to such a structure, many holecarriers are generated in an organic compound which originally hasalmost no internal carrier, and an extremely good hole injectingproperty and transporting property is obtained. According to thiseffect, a drive voltage can be lowered than a conventional case. Inaddition, the hole injecting and transporting layer can be thickenedwithout causing increase in the drive voltage, so that short-circuitingof the light-emitting element due to dust or the like can be restrained.

As the organic compound material having a hole transporting property,copper phthalocyanine (abbreviation: CuPc),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), 1,3,5-tris[N,N-di(m-tolyl)amino]benzene(abbreviation: m-MTDAB),N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine(abbreviation: TPD), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB),4,4′-bis{N-[4-di(m-tolyl)amino]phenyl-N-phenylamino}biphenyl(abbreviation: DNTPD), and the like can be given as the examples;however, the present invention is not limited thereto.

As the inorganic compound material showing an electron-acceptingproperty, titanium oxide, zirconium oxide, vanadium oxide, molybdenumoxide, tungsten oxide, rhenium oxide, ruthenium oxide, zinc oxide, andthe like can be given as the example. Especially vanadium oxide,molybdenum oxide, tungsten oxide, and rhenium oxide are preferablebecause it is possible to perform vacuum deposition and thus easilydealt with.

The electron injecting and transporting layer is formed by using anorganic compound material having an electron transporting property.Specifically, tris(8-quinolinolato)aluminum (abbreviation: Alq₃),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), bathocuproin (abbreviation: BCP),2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), and the like can be given; however, the presentinvention is not limited thereto.

As the EL layer, 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA),4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), coumarin 30,coumarin 6, coumarin 545, coumarin 545T, rubrene,2,5,8,11-tetra(tert-butyl)perylene (abbreviation: TBP),9,10-diphenylanthracene (abbreviation: DPA), 5,12-diphenyltetracene,4-(dicyanomethylene)-2-methyl-[p-(dimethylamino)styryl]-4H-pyran(abbreviation: DCM1),4-(dicyanomethylene)-2-methyl-6-[2-(julolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCM2), and the like can be given. In addition, a compoundwhich can emit phosphorescence such asbis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(picolinate)(abbreviation: Ir(CF₃ ppy)₂(pic)),tris(2-phenylpyridinate-N,C^(2′))iridium (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinate-N,C^(2′))iridium(acetylacetonato) (abbreviation:Ir(ppy)₂(acac)),bis[2-(2′-thienyl)pyridinato-N,C^(3′)iridium(acetylacetonato)(abbreviation: Ir(thp)₂(acac)), orbis(2-phenylquinolinate-N,C^(2′))iridium(acetylacetonato) (abbreviation:Ir(pq)₂(acac)) can also be used.

In addition, a singlet excited light-emitting material and a tripletexcited light-emitting material including a metal complex or the likemay be used for the EL layer. For example, among a pixel for red lightemission, a pixel for green light emission, and a pixel for blue lightemission, the pixel for red light emission of which the time ofluminance reduction by half is relatively short is formed of the tripletexcited light-emitting material, and the others are formed of thesinglet excited light-emitting material. Since the luminous efficiencyof the triplet excited light-emitting material is good, lower powerconsumption is needed to obtain the same luminance. In other words, whenapplied to the red pixel, a smaller amount of the current flown to thelight-emitting element is needed, so that the reliability can beimproved. For lower power consumption, the pixel for red light emissionand the pixel for green light emission may be formed of the tripletexcited light-emitting material and the pixel for blue light emissionmay be formed of the singlet excited light-emitting material. By formingthe green light-emitting element for which human visibility is high alsoby the triplet excited light-emitting material, the power consumptioncan be further reduced.

The EL layer may have a structure in which light-emitting layers withdifferent emission wavelengths are formed for each pixel so that colordisplay is performed. Typically, light-emitting layers corresponding tocolors of R (red), G (green) and B (blue) respectively are formed. Inthis case, by making a structure where a filter which transmits light ofthe emission wavelength is provided on the light emission side of thepixel, color purity can be improved and the pixel portion can beprevented from becoming a mirror surface (reflective). By providing thefilter, a circularly polarizing plate which is conventionally necessarycan be omitted, and it becomes possible to have no loss of light emittedfrom the light-emitting layer. Furthermore, change in the color tonewhich occurs when the pixel portion (the display screen) is seen from anoblique direction can be decreased.

By combining the pixel with the structure shown in FIG. 36 and anexternal light strength detector, light-emitting time of thelight-emitting element is changed and the luminance of the displayscreen can be controlled. Furthermore, by controlling light emission ofthe light-emitting element by the external light strength detector, thelighting time does not increase uselessly, so that power consumption ofthe display panel can be decreased and the lifetime can be extended.

Note that, as a transistor, not only a transistor using polysilicon butalso a transistor using amorphous silicon for a semiconductor layer mayalso be used.

Subsequently, a case where an amorphous silicon (a-Si:H) film is used asa semiconductor layer of a transistor will be explained. FIGS. 37A and37B show transistors having a top gate structure while FIGS. 38A and 38Band FIGS. 39A and 39B show transistors having a bottom gate structure.

FIG. 37A is a cross sectional diagram of a transistor having a top gatestructure using amorphous silicon as its semiconductor layer. As shownin FIG. 37A, a base film 2802 is formed over a substrate 2801. A pixelelectrode 2803 is formed over the base film 2802. In addition, a firstelectrode 2804 is formed from the same material and formed in the samelayer as the pixel electrode 2803.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, or the like can be used. In addition, the base film 2802 canbe formed using a single layer of aluminum nitride (AlN), silicon oxide(SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like, or a stackedlayer thereof.

A wiring 2805 and a wiring 2806 are formed over the base film 2802, andan end portion of the pixel electrode 2803 is covered with the wiring2805. An N-type semiconductor layer 2807 and an N-type semiconductorlayer 2808 each having N-type conductivity are formed over the wiring2805 and the wiring 2806. In addition, a semiconductor layer 2809 isformed between the wiring 2805 and the wiring 2806 over the base film2802, which is partially extended to the N-type semiconductor layer 2807and the N-type semiconductor layer 2808. Note that this semiconductorlayer is formed using an amorphous semiconductor film such as amorphoussilicon (a-Si:H) or a microcrystalline semiconductor (μ-Si:H). Inaddition, a gate insulating film 2810 is formed over the semiconductorlayer 2809, and an insulating film 2811 is formed from the same materialand formed in the same layer as the gate insulating film 2810 over thefirst electrode 2804. Note that a silicon oxide film, a silicon nitridefilm, or the like is used as the gate insulating film 2810.

A gate electrode 2812 is formed over the gate insulating film 2810. Inaddition, a second electrode 2813 is formed from the same material andformed in the same layer as the gate electrode over the first electrode2804 with the insulating film 2811 interposed therebetween. A capacitorelement 2819 is formed by sandwiching the insulating film 2811 betweenthe first electrode 2804 and the second electrode 2813. An interlayerinsulating film 2814 is formed to cover an end portion of the pixelelectrode 2803, a driving transistor 2818, and the capacitor 2819.

A layer 2815 containing an organic compound and an opposite electrode2816 are formed over the interlayer insulating film 2814 and the pixelelectrode 2803 corresponding to an opening of the interlayer insulatingfilm 2814. A light-emitting element 2817 is formed in a region where thelayer 2815 containing an organic compound is sandwiched between thepixel electrode 2803 and the opposite electrode 2816.

The first electrode 2804 in FIG. 37A may be a first electrode 2820 asshown in FIG. 37B. The first electrode 2820 is formed from the samematerial and formed in the same layer as the wirings 2805 and 2806.

FIGS. 38A and 38B are each a partial cross sectional diagram of a panelof a display device provided with a transistor having a bottom gatestructure using amorphous silicon as its semiconductor layer.

A base film 2902 is formed over a substrate 2901. A gate electrode 2903is formed over the base film 2902. In addition, a first electrode 2904is formed from the same material and formed in the same layer as thegate electrode. As a material for the gate electrode 2903,polycrystalline silicon added with phosphorus can be used. Besidespolycrystalline silicon, silicide that is a compound of metal andsilicon may be used as well.

Then, a gate insulating film 2905 is formed to cover the gate electrode2903 and the first electrode 2904. The gate insulating film 2905 isformed using a silicon oxide film, a silicon nitride film, or the like.

A semiconductor layer 2906 is formed over the gate insulating film 2905.In addition, a semiconductor layer 2907 is formed from the same materialand formed in the same layer as the semiconductor layer 2906.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, or the like can be used. The base film 2902 can be formedusing a single layer of aluminum nitride (AlN), silicon oxide (SiO₂),silicon oxynitride (SiO_(x)N_(y)), or the like, or a stacked layerthereof.

N-type semiconductor layers 2908 and 2909 each having N-typeconductivity are formed over the semiconductor layer 2906 while anN-type semiconductor layer 2910 is formed over the semiconductor layer2907.

Wiring 2911 and 2912 are formed over the N-type semiconductor layers2908 and 2909, respectively, while a conductive layer 2913 is formedfrom the same material and formed in the same layer as the wirings 2911and 2912, over an N-type semiconductor layer 2910.

The semiconductor layer 2907, the N-type semiconductor layer 2910, andthe conductive layer 2913 constitute a second electrode. Note that acapacitor element 2920 is formed by sandwiching the gate insulating film2905 between the second electrode and the first electrode 2904.

One end portion of the wiring 2911 is extended, and a pixel electrode2914 is formed to be in contact with the upper portion of the extendedwiring 2911.

An insulator 2915 is formed so as to cover end portions of the pixelelectrode 2914, a driving transistor 2919, and the capacitor element2920.

Then, a layer 2916 containing an organic compound and an oppositeelectrode 2917 are formed over the pixel electrode 2914 and theinsulator 2915. A display element 2918 is formed in a region where thelayer 2916 containing an organic compound is sandwiched between thepixel electrode 2914 and the opposite electrode 2917.

The semiconductor layer 2907 and the N-type semiconductor layer 2910which form part of the second electrode of the capacitor element may notbe provided. In other words, the second electrode may be constituted bythe conductive layer 2913, so that the capacitor element may be formedby sandwiching the gate insulating film between the first electrode 2904and the conductive layer 2913.

Note that, by forming the pixel electrode 2914 before forming the wiring2911 in FIG. 38A, a capacitor element 2922 can be formed by sandwichingthe gate insulating film 2905 between a second electrode 2921 which isformed of the pixel electrode 2914 and the first electrode 2904 as shownin FIG. 38B.

Note that FIGS. 38A and 38B show reverse staggered channel etch typetransistors; however, a channel protective type transistor may be used.The case of a channel protective type transistor will be explained withreference to FIGS. 39A and 39B.

A channel protective type transistor shown in FIG. 39A is different fromthe driving transistor of the channel etch type 2919 shown in FIG. 38Ain that an insulator 3001 which is to be an etching mask is providedover the channel forming region in the semiconductor layer 2906. Theother portions identical to FIG. 38A are denoted by the same referencenumerals.

Similarly, a channel protective type transistor shown in FIG. 39B isdifferent from the driving transistor of the channel etch type 2919shown in FIG. 38B in that the insulator 3001 which is to be an etchingmask is provided over the channel forming region in the semiconductorlayer 2906. The other portions identical to FIG. 38B are denoted by thesame reference numerals.

By using an amorphous semiconductor film as a semiconductor layer (achannel forming region, a source region, a drain region, and the like)of a transistor configuring the pixel of the present invention,manufacturing cost can be reduced. For example, by employing the pixelconfiguration shown in FIGS. 6A and 6B or FIG. 36 the amorphoussemiconductor film can be used.

Note that a structure of a transistor and a structure of a capacitorelement applicable in the pixel configuration of the present inventionare not limited to the above structures, and various structures can beused.

Note that the content described in this embodiment mode can bearbitrarily combined with the content described in Embodiment Modes 1 to8.

Embodiment Mode 10

The photo-sensor which detects external light strength may beincorporated into a display device. The photo-sensor may be mounted onthe display device as part, or may be formed being united with a displaypanel. In the case where it is formed being united with the displaypanel, the display surface can be used as an acceptance surface of thephoto-sensor, which has a great effect in design. In other words, grayscale control based on the external light strength can be performedwithout making users conscious of the photo-sensor attached to thedisplay device.

FIG. 40 is a diagram showing one mode in which the photo-sensor isformed united over the display panel. In FIG. 40, the case where a pixelis constituted by a light-emitting element for light-emission ofelectroluminescence and a TFT which controls the operation of thelight-emitting element is shown.

In FIG. 40, a driving TFT 8801 formed over a light-transmittingsubstrate 8800, a first electrode 8802 (a pixel electrode) formed from atransparent material, an EL layer 8803 and a second electrode 8804 (anopposite electrode) formed from a transparent material are provided. Thefirst electrode 8802 (a pixel electrode) is formed over an insulatingfilm 8841. A light-emitting element 825 emits light upward (the arrowdirection). Then, over an insulating film 8812 formed over the secondelectrode 8804, a photoelectric conversion element 8838 formed of alaminated body of a p-type layer 8831; an i-type layer 8832 which isvirtually intrinsic; and an n-type layer 8833, an electrode 8830connected to the p-type layer 8831, and an electrode 8834 connected tothe n-type layer 8833 are provided. Note that the photoelectricconversion element 8838 may be formed over the insulating film 8841.

In this embodiment mode, the photoelectric conversion element 8838 isused as a photo-sensor element. The light-emitting element 8825 and thephotoelectric conversion element 8838 are formed over the same substrate8000, and the light emitted from the light-emitting element 8825constitutes a projected image, which is viewed by the user. On the otherhand, the photoelectric conversion element has functions of detectingexternal light and sending the detection signal to a controller. In sucha manner, the light-emitting element and the photo-sensor (thephotoelectric conversion element) can be formed over the same substrate,which contributes to miniaturization of the set.

Note that the content described in this embodiment mode can bearbitrarily combined with the content described in Embodiment Modes 1 to9.

Embodiment Mode 11

In this embodiment mode, description is made on hardware for controllingthe driving methods described in Embodiment Modes 1 to 10.

FIG. 26 is a rough constitution diagram. A pixel array 2704 is arrangedover a substrate 2701. In addition, a source driver 2706 and a gatedriver 2705 are arranged in many cases. Besides, a power supply circuit,a pre-charge circuit, a timing generation circuit, or the like may bearranged. There is also a case where the source driver 2706 and the gatedriver 2705 are not arranged. In this case, a circuit which is notprovided over the substrate 2701 is formed on an IC in many cases. TheIC is mounted on the substrate 2701 by COG (Chip On Glass) in manycases. Alternatively, the IC may be mounted on a connecting substrate2707 which connects a peripheral circuit substrate 2702 and thesubstrate 2701.

A signal 2703 is input to the peripheral circuit substrate 2702, and acontroller 2708 performs control to store the signal in a memory 2709, amemory 2710, or the like. In a case where the signal 2703 is an analogsignal, the signal is stored in the memory 2709, the memory 2710, or thelike in many cases after analog-digital conversion is performed. Then,the controller 2708 outputs a signal to the substrate 2701 by using thesignal stored in the memory 2709, the memory 2710, or the like.

In order to realize the driving methods described in Embodiment Modes 1to 9, the controller 2708 controls various pulse signals or the like andoutputs a signal to the substrate 2701.

Note that the content described in this embodiment mode can bearbitrarily combined with the content described in Embodiment Modes 1 to9.

Embodiment Mode 12

A structure example of a cellular phone having a display deviceaccording to the present invention or a display device using the drivingmethod according to the present invention in a display portion will beexplained with reference to FIG. 27.

A display panel 5410 is incorporated in a housing 5400 so as to bedetachable. The shape and size of the housing 5400 can be appropriatelychanged in accordance with the size of the display panel 5410. Thehousing 5400 to which the display panel 5410 is fixed is fitted in aprinted wiring board 5401 and assembled as a module.

The display panel 5410 is connected to the printed wiring board 5401through an FPC 5411. The printed wiring board 5401 is provided with aspeaker 5402, a microphone 5403, a transmitting and receiving circuit5404, and a signal processing circuit 5405 including a CPU, acontroller, and the like. Such a module, an input means 5406, and abuttery 5407 are combined and stored using a chassis 5409. Note that apixel portion of the display panel 5410 is arranged so as to be seenfrom a window formed in the chassis 5412.

In the display panel 5410, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed using TFTs in anintegrated manner over a substrate, and another part of the peripheraldriver circuits (a driver circuit having a high operation frequencyamong the plurality of driver circuits) may be formed on an IC chip. TheIC chip may be mounted on the display panel 5410 by COG (Chip On Glass).The IC chip may alternatively be connected to a glass substrate usingTAB (Tape Automated Bonding) or a printed wiring board. Note that FIG.28A shows an example of a configuration of a display panel where part ofperipheral driver circuits is integrated with a pixel portion over asubstrate and an IC chip on which the another part of peripheral drivercircuits is formed is mounted by COG or the like. The display panelshown in FIG. 28A has a substrate 5300, a signal line driver circuit5301, a pixel portion 5302, first scanning line driver circuit 5303,second scanning driver circuit 5304, an FPC 5305, IC chips 5306 and5307, a sealing substrate 5308, a sealing material 5309. The signal linedriver circuit 5301 which is formed in an IC chip is mounted over thesubstrate 5300 as a COG or the like. By using the above configuration,power consumption of the display device can be reduced, and operatingtime per charge of a cellular phone handset can be made longer. Inaddition, cost reduction of a cellular phone handset can be achieved.

In addition, by converting the impedance of a signal set to a scanningline or a signal line by using a buffer, a write period for pixels ofeach row can be shortened. Accordingly, a high-definition display devicecan be provided.

Moreover, in order to further reduce power consumption, a pixel portionmay be formed using TFTs over a glass substrate, and all signal linedriver circuits may be formed on an IC chip, which may be mounted on adisplay panel by COG (Chip On Glass) or the like.

By using a display device of the present invention, it becomes possibleto see a high-contrast clear image.

The structure described in this embodiment mode is an example of acellular phone, and a display device according to the present inventioncan be applied not only to the cellular phone having the above structurebut also to cellular phones having various kinds of structures.

Embodiment Mode 13

FIG. 29 shows an EL module in which a display panel 5701 and a circuitboard 5702 are combined. The display panel 5701 includes a pixel portion5703, a scanning line driver circuit 5704, and a signal line drivercircuit 5705. Over the circuit board 5702, for example, a controlcircuit 5706, a signal dividing circuit 5707, and the like are formed.The display panel 5701 and the circuit board 5702 are connected to eachother by a connection wiring 5708. As the connection wiring, an FPC orthe like can be used.

The control circuit 5706 corresponds to the controller 2708, the memory2709, the memory 2710, or the like in Embodiment Mode 7. Mainly in thecontrol circuit 4706, the appearance order of sub-frames or the like iscontrolled.

In the display panel 5701, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed using TFTs in anintegrated manner over a substrate, and another part of the peripheraldriver circuits (a driver circuit having a high operation frequencyamong the plurality of driver circuits) may be formed on an IC chip. TheIC chip may be mounted on the display panel 5701 by COG (Chip On Glass)or the like. The IC chip may alternatively be mounted on the displaypanel 5701 by using TAB (Tape Automated Bonding) or a printed wiringboard. Note that FIG. 28A shows an example of a configuration of adisplay panel where part of peripheral driver circuits is integratedwith a pixel portion over a substrate and an IC chip on which anotherpart of the peripheral driver circuits is formed is mounted by COG orthe like. By using the above configuration, power consumption of thedisplay device can be reduced, and operating time per charge of acellular phone handset can be made longer. In addition, cost reductionof a cellular phone handset can be achieved.

In addition, by converting the impedance of a signal set to a scanningline or a signal line by using a buffer, a write period for pixels ofeach row can be shortened. Accordingly, a high-definition display devicecan be provided.

Moreover, in order to further reduce power consumption, a pixel portionmay be formed using TFTs over a glass substrate, and all signal linedriver circuits may be formed on an IC chip, which may be mounted on adisplay panel by COG (Chip On Glass) or the like. The display panelshown in FIG. 28B has a substrate 5310, a signal line driver circuit5311, a pixel portion 5312, first scanning line driver circuit 5313,second scanning driver circuit 5314, an FPC 5315, IC chips 5316 and5317, a sealing substrate 5318, a sealing material 5319. The signal linedriver circuit 5311, the first scanning driver circuit 5313 and thesecond scanning driver circuit 5314 which are formed in IC chips aremounted over the substrate 5310 as a COG or the like.

Note that a pixel portion may be formed using a TFT over a substrate,and all peripheral driver circuits may be formed on an IC chip, whichmay be mounted on a display panel by COG (Chip On Glass). Note that FIG.28B shows an example of constitution where a pixel portion is formedover a substrate and an IC chip provided with a signal line drivercircuit is mounted on the substrate by COG or the like.

An EL television receiver can be completed with the above EL module.FIG. 30 is a block diagram showing a main constitution of an ELtelevision receiver. A tuner 5801 receives a video signal and an audiosignal. The video signal is processed by a video signal amplifiercircuit 5802, a video signal processing circuit 5803 for converting asignal output from the video signal amplifier circuit 5802 into a colorsignal corresponding to each color of red, green and blue, and thecontrol circuit 5706 for converting the video signal into the inputspecification of a driver circuit. The control circuit 5706 outputsrespective signals to the scanning line side and the signal line side.In a case of driving in a digital manner, a configuration in which thesignal dividing circuit 5707 is provided on the signal line side tosupply an input digital signal divided into m pieces may be adopted.

The audio signal among the signals received by the tuner 5801 istransmitted to an audio signal amplifier circuit 5804, an output ofwhich is supplied to a speaker 5806 through an audio signal processingcircuit 5805. A control circuit 5807 receives control information of areceiving station (reception frequency) or sound volume from an inputportion 5808 and transmits signals to the tuner 5801 and the audiosignal processing circuit 5805.

By incorporating the EL module into a chassis, a television receiver canbe completed. A display portion is formed with the EL module. Inaddition, a speaker, a video input terminal, or the like are providedappropriately.

Of course, the present invention is not limited to the televisionreceiver, and can be applied to various uses as a large-sized displaymedium such as an information display board at a train station, anairport, or the like, or an advertisement display board on the street,as well as a monitor of a personal computer.

By using the display device of the present invention as described above,it becomes possible to see a high-contrast clear image.

Note that the content described in this embodiment mode can bearbitrarily combined with the content described in Embodiment Modes 1 to13.

Embodiment Mode 14

In this embodiment mode, examples of a photo-sensor and an amplifierwill be shown.

FIG. 44 shows a basic configuration diagram. A photoelectric conversionelement 3601 is irradiated with light and current flows depending onilluminance. The current is converted into a voltage signal by acurrent-voltage conversion circuit 3902. In this manner, a photo-sensor113 is formed to include the photoelectric conversion element 3601 andthe current-voltage conversion circuit 3902. Then the signal outputtedfrom the photo-sensor 113 is inputted into an amplifier 114. FIG. 44shows a voltage follower circuit using an operational amplifier.However, the present invention is not limited thereto.

As shown in FIG. 41, a resistive element 3602 may be used as an exampleof the current-voltage conversion circuit 3902. However, the presentinvention is not limited thereto. The circuit may also be formed usingan operational amplifier.

Although current flowing to the photoelectric conversion element 3601 isused in FIGS. 44 and 41, the current may also be amplified. For example,as shown in FIG. 42, current flowing to a resistive element 3702, whichis a current-voltage conversion circuit, may be increased using acurrent mirror circuit 3703. Consequently, it is possible to improvesensitivity to light or to improve noise immunity.

In addition, as shown in FIG. 43, all current flowing to thephotoelectric conversion element 3601 and the current mirror circuit3803 may be made to flow to the current-voltage conversion circuit 3802so that, further, sensitivity to light is improved or resistance tonoise is improved. Accordingly, it is possible to a wiring connected tothe photoelectric conversion element 3601 and the output of the currentmirror circuit can be made into one; thus, the number of connectionterminals can be reduced.

Note that the content described in this embodiment mode can bearbitrarily combined with the content described in Embodiment Modes 1 to14.

Embodiment Mode 15

The present invention can be applied to various electronic devices.

Specifically, it can be applied to a display portion of an electronicdevice. Examples of such an electronic device are as follows: a camerasuch as a video camera or a digital camera, a goggle type display, anavigation system, an audio reproducing device (such as a car audio oran audio component), a computer, a game machine, a portable informationterminal (such as a mobile computer, a cellular phone, a portable gamemachine, or an electronic book), an image reproducing device providedwith a recording medium reading portion (specifically, a device whichcan reproduce a recording medium such as a digital versatile disc (DVD)and includes a light-emitting device capable of displaying imagesthereof), and the like.

FIG. 31A shows a light-emitting device, which includes a chassis 35001,a supporting stand 35002, a display portion 35003, a speaker portion35004, a video input terminal 35005, and the like. The display device ofthe present invention can be used for the display portion 35003. Notethat the light-emitting device includes in its category alllight-emitting devices used for displaying information, for example, fora personal computer, for TV broadcast reception, or for advertisementdisplay. The light-emitting device using the present invention for thedisplay portion 35003 makes it possible to see a high-contrast clearimage.

FIG. 31B shows a camera, which includes a main body 35101, a displayportion 35102, an image receiving portion 35013, an operation key 35104,an external connection port 35105, a shutter 35106, and the like.

The camera using the present invention for the display portion 35102makes it possible to see a high-contrast clear image.

FIG. 31C shows a computer, which includes a main body 35201, a chassis35202, a display portion 35203, a keyboard 35204, an external connectionport 35205, a pointing mouse 35206, and the like. The computer using thepresent invention for the display portion 35203 makes it possible to seea high-contrast clear image.

FIG. 31D shows a mobile computer, which includes a main body 35301, adisplay portion 35302, a switch 35303, operation keys 35304, an infraredport 35305, and the like. The mobile computer using the presentinvention for the display portion 35302 makes it possible to see ahigh-contrast clear image.

FIG. 31E shows a portable image reproducing device provided with arecording medium reading portion (specifically, a DVD reproducingdevice), which includes a main body 35401, a chassis 35402, a displayportion A 35403, a display portion B 35404, a recording medium (DVD orthe like) reading portion 35405, operation keys 35406, a speaker portion35407, and the like. The display portion A 35403 mainly displays imageinformation, and the display portion B 35404 mainly displays characterinformation. The image reproducing device using the present inventionfor the display portion A 35403 and the display portion B 35404 makes itpossible to see a high-contrast clear image.

FIG. 31F shows a goggle type display, which includes a main body 35501,a display portion 35502, an arm portion 35503, and the like. The goggletype display using the present invention for the display portion 35502makes it possible to see a high-contrast clear image.

FIG. 31G shows a video camera, which includes a main body 35601, adisplay portion 35602, a chassis 35603, an external connection port35604, a remote control receiving portion 35605, an image receivingportion 35606, a battery 35607, an audio input portion 35608, operationkeys 35609, an eye piece portion 35610, and the like. The video camerausing the present invention for the display portion 35602 makes itpossible to see a high-contrast clear image.

FIG. 31H shows a cellular phone handset, which includes a main body35701, a chassis 35702, a display portion 35703, an audio input portion35704, an audio output portion 35705, operation keys 35706, an externalconnection port 35707, an antenna 35708, and the like. The cellularphone handset using the present invention for the display portion 35703makes it possible to see a high-contrast clear image.

As described above, the applicable range of the present invention is sowide that the present invention can be applied to electronic devices ofvarious fields. In addition, the electronic device of this embodimentmode may use a display device having any of the structures described inEmbodiment Modes 1 to 14.

The present application is based on Japanese Patent Application SerialNo. 2005-148837 filed on May 20, 2005 in Japanese Patent Office, theentire contents of which are hereby incorporated by reference.

TABLE 1 Brightness (lux) Rough Indication of Brightness (lux) 1,000,000Toyama Beach in midsummer >100,000 Sunlight of Sunny day in Daytime100,000 Sunlight of Sunny day at 10 a.m. 65,000 Sunlight of Sunny day at3 p.m. 35,000 Sunlight of Cloudy day in Daytime 32,000 Sunlight ofCloudy day at 10 a.m. 25,000 10,000 Sunlight of Cloudy day after 1 hour2,000 from sunrise 1,000 Sunlight of Sunny day at 1 hour 1,000 beforesunset Lighting of Pachinko Parlors 1,000 Lighting of Department Store500-700 Fluorescent Lamp of Office 400-500 Sunlight at Sunrise/Sunset300 Two 30 W Fluorescent Lamps in eight 300 tatami mats room Arcade atnight 150-200 100 Under Fluorescent Lamp  50-100 30 cm away from Lighter15 10 20 cm away form candle 10-15 Civil Twilight (Zenith Distance of 5Sun 96 degree) 1 Moonlight 0.5-1   Nautical Twilight (Zenith Distance of0.01 Sun 102 degree) Astronomic Twilight (Zenith Distance of 0.001 Sun108 degree)

TABLE 2 ~10,000 [1x]~ ~100,000 [1x]~ 500~1,500 [1x] ←in the open air ←inthe open air Power inside room→ ←inside lighted hall→ of cloudy day→ ofsunny day→ consumption EL 2-tone Good visibility is ⊚~◯ Good visibilityis ◯ Visibility of text can be ◯~Δ ⊚ panel obtained with respect toobtained with respect to kept. (2.0 8-tone natural image and text. text.In low contrast, In low contrast, Δ ⊚ QVGA) visibility decreases, whenvisibility decreases. background color is close In low contrast, to thecontrast. Visibility decreases. natural In low contrast, visibility ΔVisibility becomes Δ~X ◯ image decreases, when deteriorated. In low(>64-tone) peripheral display part is contrast, visibility Halftone.decreases. Transmissive LCD Good visibility is ⊚~◯ Same as above. Δ~XVisibility becomes X ◯~Δ Panel (1.9QVGA) obtained with respect toVisibility of text is on deteriorated. natural image and text. equalitywith EL Panel. Sometime, viewers can However, contrast However,visibility of not have visual under decreases compared natural image hasno Direct sunshine. with that of EL panel. advantage over EL Panel.Semi-Transmissive Good visibility is ◯ Comparatively good ◯Comparatively good ◯ ◯ LCD Panel obtained with respect to visibility ofNatural visibility is kept, since (2.1QCIF+) natural image and text.Image is obtained. reflection component of However, contrast Contrastdoes not external light increases. decreases compared decrease. Colordoes not with that of EL panel. shift. Reflection LCD Visibilitydecreases Δ~X In low contrast, visibility ◯ Comparatively good ◯ ⊚ Paneleminently. In low decreases when peripheral visibility is kept, sincecontrast, visibility region is Halftone. reflection component ofdecreases. external light increases.

1. A light-emitting device comprising: a light-emitting elementincluding an EL layer between a pair of electrodes; and a photo-sensorconfigured to detect an external light, wherein the photo-sensorincludes a photoelectric conversion element and a current-voltageconversion circuit.
 2. A light-emitting device according to claim 1,wherein the photoelectric conversion element includes a first electrode,a p-type layer over the first electrode, an i-type layer over the p-typelayer, an n-type layer over the i-type layer, and a second electrodeover the n-type layer.
 3. A light-emitting device according to claim 1,wherein the photoelectric conversion element is formed so as not tooverlap the light-emitting element via an insulating film.
 4. Alight-emitting device according to claim 1, further comprising a displaymode switching control circuit configured to switch between an analoggray scale method and a digital gray scale method.
 5. A light-emittingdevice according to claim 1, wherein the light-emitting device isincorporated in one selected from the group consisting of a camera, acomputer, a goggle-type display, an image reproducing device, and aphone.
 6. A light-emitting device comprising: a light-emitting elementincluding an EL layer between a pair of electrodes; a photo-sensorconfigured to detect an external light, wherein the photo-sensorincludes a photoelectric conversion element and a current-voltageconversion circuit; and an amplifier whose input is connected to anoutput of the current-voltage conversion circuit.
 7. A light-emittingdevice according to claim 6, wherein the photoelectric conversionelement includes a first electrode, a p-type layer over the firstelectrode, an i-type layer over the p-type layer, an n-type layer overthe i-type layer, and a second electrode over the n-type layer.
 8. Alight-emitting device according to claim 6, wherein the photoelectricconversion element is formed so as not to overlap the light-emittingelement via an insulating film.
 9. A light-emitting device according toclaim 6, further comprising a display mode switching control circuitconfigured to switch between an analog gray scale method and a digitalgray scale method.
 10. A light-emitting device according to claim 6,wherein the light-emitting device is incorporated in one selected fromthe group consisting of a camera, a computer, a goggle-type display, animage reproducing device, and a phone.
 11. A light-emitting devicecomprising: a light-emitting element including an EL layer between apair of electrodes; and a photo-sensor configured to detect an externallight, wherein the photo-sensor includes a current mirror circuit, aphotoelectric conversion element and a current-voltage conversioncircuit.
 12. A light-emitting device according to claim 11, wherein thephotoelectric conversion element includes a first electrode, a p-typelayer over the first electrode, an i-type layer over the p-type layer,an n-type layer over the i-type layer, and a second electrode over then-type layer.
 13. A light-emitting device according to claim 11, whereinthe photoelectric conversion element is formed so as not to overlap thelight-emitting element via an insulating film.
 14. A light-emittingdevice according to claim 11, further comprising a display modeswitching control circuit configured to switch between an analog grayscale method and a digital gray scale method.
 15. A light-emittingdevice according to claim 11, wherein the light-emitting device isincorporated in one selected from the group consisting of a camera, acomputer, a goggle-type display, an image reproducing device, and aphone.
 16. A light-emitting device comprising: a light-emitting elementincluding an EL layer between a pair of electrodes; a photo-sensorconfigured to detect an external light, wherein the photo-sensorincludes a current mirror circuit, a photoelectric conversion elementand a current-voltage conversion circuit; and an amplifier whose inputis connected to an output of the current-voltage conversion circuit. 17.A light-emitting device according to claim 16, wherein the photoelectricconversion element includes a first electrode, a p-type layer over thefirst electrode, an i-type layer over the p-type layer, an n-type layerover the i-type layer, and a second electrode over the n-type layer. 18.A light-emitting device according to claim 16, wherein the photoelectricconversion element is formed so as not to overlap the light-emittingelement via an insulating film.
 19. A light-emitting device according toclaim 16, further comprising a display mode switching control circuitconfigured to switch between an analog gray scale method and a digitalgray scale method.
 20. A light-emitting device according to claim 16,wherein the light-emitting device is incorporated in one selected fromthe group consisting of a camera, a computer, a goggle-type display, animage reproducing device, and a phone.